Is a name classed as a word

Every programmer agrees naming classes is highly important for code readability. Proper naming decrease the time needed to understand the code base. Many developers will also agree that class naming isn’t easy. There are numerous queries around the best practices which come not only from the beginners.

The aim of this article is to put in one place answers for the most popular questions around Java class name conventions and community standards. I’ll cover technical Java language restrictions, common conventions, and popular class naming best practices.

So much by way of introduction. Let’s get down into it.

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Class name rules and restrictions

When it comes to technical limitations for class names, the Java language is pretty flexible. But realistically, programmers rarely use this flexibility.

Java class max length

The specification of the programming language states that the length of a Java class name is unlimited. In practice, however, the first limit comes from the JVM specification. For the current JVM it’s 65535 characters.

Moreover, the class name is usually the same as the filename in which you define the type. Because of that, there is one more factor affecting the max length of the class name. It’s Operating System and underlying filesystem.

In Windows, the limit for a single filename is 260 characters. The .java file extension takes 5 characters which leaves us with 255 character limit for class names. For UNIX based system most file systems also restrict the length of a file to 255 characters.

Characters allowed in Java class name

The language specification states that a class name should be a sequence of so-called Java letters or Java digits. The first character should be a Java letter.

What is a Java letter? Or Java digit?

A Java letter is a set of Unicode characters, underscore (_), and dollar sign ($). A Java digit is a collection of numbers from 0 to 9

The idea behind the Unicode characters as a valid part of Java class names is to allow programming in the native language of developers all over the world. However, this isn’t commonly practiced. Usually, programmers limit themselves to the English alphabet.

Once I’ve seen an app write in my native language and it was a pretty hilarious experience. Especially some plural variable names generated by IDE.

Java class name conventions and standards

Java is already a pretty old programming language. Over the years, developers created several commonly agreed standards for naming classes. These standards are much more restrictive than limitations from the Java language specification. Naming conventions allow other to read your code easily.

Please note almost all naming conventions are opinion based and not technically imposed. You may meet developers who disagree with some of the presented conventions (especially if they have a background in different programming languages). However, you definitely should discuss and agree on standards your team will follow inside a single project.

Below you can find the list of answers for the most popular questions around class naming conventions. If something in your opinion is missing on the list, please let me know about it in a comment.

Class name should be noun

In Object Oriented Programming, a class represents an existing entity from the real world that you model in your application. Classes describe things. Just think about classic examples like User, Message, or Event.

But single word class names are rather rare. Usually, we use noun phrases to provide more descriptive names for classes such as AnonymousUser, DirectMessage, or UserCreatedEvent. The less abstract a class is, the more details are present in its name.

You should reserve verbs for method names as they represent actions executed by things.

Singular or plural nouns?

When you think of a class as a template for objects, the choice should be obvious. In almost all cases objects we create represent singular entities. Just one item of something.

When we work with multiple instances, we usually use collections which are available in the Java Class Library. Collections always use singular nouns for their names. Think of List, Set, or Map to name a few. While working with multiple instances of some class, we usually manipulate them with some kind of singular container.

But hold on a second.

Don’t we have counterexamples inside JDK? What about classes like Objects, Arrays or Collections?

Unlike most classes we create, these JDK classes don’t represent any entities. What they all have in common is that you can’t create an instance of them. Try calling Objects constructor if you need proof. These types are just containers for static methods. These utility methods don’t belong to any particular instance.

On the other side of the coin, JDK has the String class which you can instantiate and which also contains static utility methods. It seems like an inconsistency in the design of JDK and maybe that’s why we’re all confused.

When it comes to static methods, it’s just a matter of choice. You can put such methods in a class with a singular name or create a plural named class as a namespace. As long as your team is consistent, you can’t go wrong with either option.

UserDao vs. UsersDao

The dilemma from the above subtitle is probably popular only amongst non-native English speaker (like myself). It extends on the previous point about singular and plural nouns in class names.

Imagine you have a class which manipulates multiple instances of some type, e.g. a repository class for the database access layer. Should you use a plural or a singular form of that type?

Let me answer the question with the picture:

Do you know the English name of this tool?

It’s a screwdriver.

You can screw multiple screws with a screwdriver. Yet, in the name of the tool, there is only a singular screw. Can you see the analogy for the DAO class?

Class name same as filename

The Java compiler expects that the class name matches the filename in which it’s defined. However, this requirement is true only for public classes.

You technically can define a non-public class in a file with a different name or even write several classes in a single file. The Java compiler won’t prevent you from doing this.

But just because you can name files differently than classes, it doesn’t mean you should.

Java programmers commonly agree the class name and the filename should be the same. Having only a single class defined in a file is also considered as a standard. Such an approach is simpler for navigation around project’s file structure.

Class name starts with capital letter

As I already mentioned, a class name must start with a Java letter. The language specification sets this requirement. But, it doesn’t specify the case of this letter.

It’s just a common agreement amongst Java developers for easy readability of the code to start class names with a capitalized letter. When you create a class name with several words, you should capitalize each word. The upper case letter acts as a separator.

Can Java class name contain underscore?

Technically it’s possible to use the underscore in a class name, but there’s no practical reason to do so.

If the underscore were used in a class name, it would act as a separator between words. In Java, we achieve optical separation between words with the camel case. By using the underscore, we would only unnecessarily enlarged name’s length.

Class name length best practice

When a class name is too long?

A name of a class is too long when another shorter name with exactly the same meaning exists.

There’s no exact number which you should set in your static code analysis tool. The main aim of a class name is to describe the purpose of this class. The lesser short words you can use to name the class, the better for the code readability.

But why short names are so important?

Long names (not only of classes but method and variables too) affects the readability of code blocks. Vertical scroll bars in IDEs or simple line breaks are considered as distractors.

Our brain associates text lines with programming instructions. When an instruction spans multiple lines, many programmers find it hard to skim over code blocks.

Abbreviation in class name

The compiler only cares if names are unique. The code is written to be read by humans, not computers.

We already discussed short class names are important. Abbreviations seem like a way to achieve shorter names. Yet, most Java programmers prefer to avoid abbreviations.

However, some teams accept abbreviations for certain words. Especially, if these words appear in the numerous number of classes with similar purpose. An example of such word might be Config or Id. If your team belongs to this group, make sure all classes in a project follow the same rule.

Acronym in class name

An acronym is a word made from the first letters of other words. All letters in an acronym are capitalized, e.g. HTML.

Should an acronym be capitalized when used in a class name?

Unfortunately, if you check conventions in the Java Class Library, you’ll find examples for both approaches. Because of that, some people simply think it doesn’t really matter.
However, in Effective Java, Joshua Bloch suggests sticking to camel case also for acronyms. The example which illustrates benefits for readability is when a class name contains several acronyms next to each other, for instance HttpUrl. Using the upper case for both words would make the class name blurry and hard to read.

Java class naming best practices

We already discussed technical restrictions and popular naming conventions. Now it’s time to present a handful of tips which should make naming classes easier for you.

Follow Single Responsibility Principle

The most important piece of advice you can ever get for Java class naming is to understand and apply the Single Responsibility Principle.

In brief, SRP tells us that a class should have only one reason to change. The name of a class should explain this reason.

Of course, SRP doesn’t only influence class naming. It changes the whole way you design your code. When you are new to SRP, it might seem odd and redundant to create numerous small classes. Fortunately, the more you practice SRP, the more benefits you’ll see.

Start with name placeholder

When you create a class and can’t come up with a name for it right away, give it a temporary name line ClassX and start implementing its body. Once you finish, the purpose of the class should be more obvious and naming shouldn’t be a problem.

But beware:

Without a name and a clear goal for a class, you can easily create a typical God object. If after implementation you still have a problem to name a class because it does several things, it might be a sign the class should be separated into a few smaller classes.

Avoid synonyms

It very common to have several classes which have the same purpose but in a different context. For example, let’s consider classes responsible for creating different objects. We can call such class as Creator, Constructor, or Factory.

The word you select to describe the purpose is a personal preference but keep the choice consistent across your whole application. Your team newcomers will thank you. It will be easier for them to see code conventions in the project at a glance.

Naming interface implementers

If you have an interface which is implemented only by one class and the name of this class is the same as interface’s name followed by the Impl suffix, you should be aware that something is wrong. It’s highly possible this interface is totally useless and the code would be much more readable with just a class named as this interface.

When a class implements an interface, it probably has specific traits that you can reflect in the name of this class. Just look at concrete implementations of the List interface from the Java Class Library. Names like ArrayList or LinkedList give you an overview of differences between them.

Conventions for naming service classes

Naming classes which model your domain objects is usually pretty straightforward. These names come from domain experts who know business very well. But these classes are just a small part of an application.

Yet, every application needs some glue code in which we create and manipulate aforementioned business objects. Such glue code usually resides in classes called Services.

You can find a lot of bad opinions about such service classes and numerous suggestions to avoid them.

But let’s be frank.

When you try to give a structure to your application, you can’t get rid of some sort of service classes. The name you choose for the glue layer doesn’t really matter. Service, Manager, Coordinator, sometimes Controller. Whatever. Pick your poison. You need a place in your application for coordinating work on business objects.

The general idea of service classes isn’t bad when you think of them as a glue layer. The problem with this type of service classes is at the pace they grow. Programmers tend to add way too many responsibilities to these classes which make them hard to maintain God objects.

The word Service is pretty generic and maybe that’s why developers assign so many responsibilities to these classes. But this word is used in our industry for so long that I don’t think it will ever change.

But there’s one thing we can improve.

We can split the responsibilities of service classes into smaller, better named and more focused classes. In other words, we apply the already discussed Single Responsibility Principle.

Util and helper class naming convention

Unfortunately, the Java Class Library doesn’t give us a clear guideline on naming utility method holders. Once again, the word you choose doesn’t really matter as long as you’re consistent inside your project.

Having classes called AccountUtils,  Accounts, and AccountUtilities will only confuse readers. Pick one convention and stick to it.

Conclusion

At this point, you should be familiar with the most popular Java naming conventions, standards, and best practices. We went through the technical restrictions, commonly agreed opinion based naming approaches, and finally covered a few class naming hints.

I strongly encourage you to leave a comment if you think something is missing on the list of mentioned topics. I’ll glad to extend the article to make it more accurate. Different opinions are also very welcome. If you find the post useful, don’t forget to spread the word.

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Other software developers in the field of Bioinformatics might be interested in a solution of this problem, so I post here my approach as suggested by Alasdair.

The goal is to create a model for a living species, for the sake of simplicity let’s say an animal, and create an endpoint with Django REST Framework representing the correct taxonomic ranks.

models.py

from django.db import models

class Animal(models.Model):
    canonical_name = models.CharField(max_length=100, unique=True)
    species = models.CharField(max_length=60, unique=True)
    genus = models.CharField(max_length=30)
    family = models.CharField(max_length=30)
    order = models.CharField(max_length=30)
    # we can't use class as field name
    class_name = models.CharField('Class', db_column='class', max_length=30)
    phylum = models.CharField(max_length=30)
    # we don't need to define kingdom and domain
    # it's clear that it is an animal and eukaryote

    def __str__(self):
        return '{} ({})'.format(self.canonical_name, self.species)

serializers.py

from collections import OrderedDict

from rest_framework import serializers

from .models import Species

class SpeciesSerializer(serializers.HyperlinkedModelSerializer):
    class Meta:
        model = Animal
        fields = ('url', 'id', 'canonical_name', 'species', 'genus',
            'subfamily', 'family', 'order', 'class_name', 'phylum')

    def to_representation(self, obj):
        # call the parent method and get an OrderedDict
        data = super(SpeciesSerializer, self).to_representation(obj)
        # generate a list of the keys and replace the key 'class_name'
        keys = list(data.keys())
        keys.insert(keys.index('class_name'), 'class')
        keys.remove('class_name')
        # remove 'class_name' and assign its value to a new key 'class'
        class_name = data.pop('class_name')
        data.update({'class': class_name})
        # create new OrderedDict with the order given by the keys
        response = OrderedDict((k, data[k]) for k in keys)
        return response

The method to_representation helps us to manipulate the output. I have put some extra work here to get the taxonomic ranks in the desired order.

Thus for the red fox the output looks like this:

Red fox (Vulpes vulpes)

{
    "url": "http://localhost:8000/animal/1",
    "id": 1,
    "canonical_name": "Red fox",
    "species": "Vulpes vulpes",
    "genus": "Vulpes",
    "family": "Canidae",
    "order": "Carnivora",
    "class": "Mammalia",
    "phylum": "Chordata"
}

It is a simplified example and in reality you’d have many more fields or possibly a model for every taxonomic rank, but somewhere you might come across the conflict between the reserved word class and the taxonomic rank class.
I hope this can help other people too.

Words are the building blocks in any sentence. They just don’t ‘mean’ something, they ‘do’ something in every sentence. Hence words are grouped into word classes based on what they do. A word class is a group of words that have certain common features. The term “word class” is analogous to the more conventional term, “part of speech.” It is also variously named grammatical category, lexical category, and syntactic category.

• Types of Word Classes
• Open and Closed Word Classes
• Open Word Classes
• Closed Word Classes
• How to identify the word classes in a sentence?
• How to classify a word class?
• What is the difference between a word class and part of speech?

Word classes can be divided into two families:

• Lexical Classes: Also known as open classes and form classes. The lexical classes include nouns, verbs, adjectives, and adverbs.
• Function Classes: Also known as closed classes and structure classes. Includes: pronouns, determiners, conjunctions, prepositions, and interjections.

Open and Closed Word Classes

As previously mentioned some word classes are open, that is, the class can be expanded with the addition of new words. Take the example of the class of nouns, it is potentially infinite as the number of words in the class is increasing as new scientific and technological discoveries are made.

The latter half of the twentieth century witnessed developments in computer technology which have in turn given rise to many new nouns like the Internet, URL website, bitmap, email, etc.

On the other hand, the word classes of prepositions, determiners, or conjunctions are known as closed word classes. Words like of, the, and but come under these. They are named closed word classes because they consist of a definite set of words. These classes never expand even though the words included in the class may change their spelling.

Open Word Classes

1) Nouns

This class includes words that you frequently use in everyday life. Nouns are most commonly understood as “naming” words, that is, it performs the function of naming “people, places or things”.

• A person – Boy, Girl, John, etc
• A thing- House, Dog, etc
• A place- China, America, etc

However, the use of nouns is not restricted to just names of people, places, or things. Nouns also denote abstract and intangible concepts such as an idea, quality, or state. Example: Danger, Happiness, Love, etc.

2) Verbs

The words that you use to describe an action are known as verbs. Hence verbs are generally known as “action” words. Have a look at the given example: Rahul rides a scooter. The verb in the above sentence denotes an action that Rahul performs which is the action of riding a scooter.

However, the idea of verbs as “action” words is somewhat restricted. Many verbs don’t stand for action at all as in the given instance: Rahul seems desperate. We cannot say that the verb ‘seems ‘ refer to an action.

3) Adverbs

In English, an adverb describes a word that alters the meaning of a verb, adjective, or another adverb. Adverbs in a sentence give you more information about the sentence. They are used to express how an action is fulfilled. Adverbs can broadly be categorized into Simple Adverbs, IInterrogative adverbs, and Relative Adverbs.

Remember:

• Most adverbs end with the common ending – ly.
• An adverb that modifies an adjective or another adverb usually goes before it.

4) Adjectives

Adjectives describe the quality of a noun. For example They stay in a beautiful house

The word beautiful indicates or refers to one of the attributes of the house that is described. Hence beautiful becomes the adjective in the above sentence.

A point to keep in mind: Some adjectives can be identified by their ending. Typical adjective endings include: able, al, ful, ic, etc.

You can even try out our other articles on How to Improve Your Vocabulary as well to expand your knowledge base.

Closed Word Classes

1) Determiners

You might have often noticed that nouns are preceded by words like the, a, or an. These words are known as Determiners. They suggest the type of reference that the noun has.

• The determiner ‘the’ is called a Definite Article. It can be placed both before singular and plural nouns. For example The Taxi, The taxis
• The determiner a or an is known as the Indefinite Article. It is used along with a singular noun. Example: A taxi

Apart from these, many other determiners express quantity. These include ‘al’, ‘both’, ‘many’ etc.

2) Conjunctions

These are used to express connections between different words.

Example: John and David are friends. And is used as a conjunction in the given sentence.
The most familiar conjunctions in English are: and, but, and or.

Conjunctions are further divided into two:

• Coordinating Conjunctions: These conjunctions connect elements of equal syntactic structure. Example: Paul and David study together.
• Subordinating Conjunctions: Connects elements of unequal syntactic structure. Example: I left early because I had an interview the next day.

3) Prepositions

Prepositions indicate the relation between different words. They occur before a noun, pronoun, or noun phrase and indicate a direction, time, place, location, and spatial relationship. Common prepositions include across, after, at, before, by, during, from, in, into, of, on, to, under, with, without, etc.

4) Pronouns

If we did not have the pronoun word families we would have to repeat a whole lot of nouns. A word that takes the position of a noun is named as a pronoun. Pronouns can be employed as a substitute for a noun.

• Pronouns are divided into 5 categories:
• Personal Pronouns: I, you, she, etc
• Demonstrative Pronouns: This, these, etc
• Possessive Pronouns: Yours, His, etc
• Interrogative Pronouns: Which, What, etc
• Reflexive Pronouns: Herself, Himself, etc.
• Reciprocal Pronouns: Each other
• Indefinite Pronouns: Few, Nobody, etc.
• Relative Pronouns: Which, Whom, etc.

5) Interjections

Short exclamations like Oh!, Ah! etc are known as Interjections. Even though they have no grammatical value, we often use them in daily speech. Interjections are primarily used to express emotions such as anger, surprise, etc. Given below are a few examples.

Well! That hurts
Hey! Don’t be so clumsy

Remember, an interjection is always followed by an exclamation mark.

Read More:

• English Idioms
• Literary Devices

FAQs on Word Classes

1. How to identify the word classes in a sentence?

A word class is a group of words that have certain common features. To find out the word classes within a sentence it is important that you familiarise yourself with the most common word classes in English. These include nouns, adjectives, verbs, adverbs, prepositions, etc.

2. How to classify a word class?

Word classes in English belong to two major categories. These are Open word classes that include nouns, verbs, adjectives, and adverbs. The second category is closed word classes that include: pronouns, determiners, interjections, etc.

3. What is the difference between a word class and part of speech?

The term “word class” is analogous to the more conventional term, “part of speech”. Both these terms refer to a group of words that have certain common features.

Conclusion

To understand the grammatical structures of sentences in a better way it’s best if you begin with word classes. Even though comprehending the different word classes may initially be a hectic task, once you master word classes, you will reach the exact meaning or message conveyed by a sentence.

The words
of language, depending on various formal and semantic features,
are divided into grammatically relevant sets or classes. The
tra­ditional grammatical classes of words are called «parts
of speech». Since the
word is distinguished not only by grammatical, but also by
semantico-lexemic properties, some scholars refer to parts of speech
as «lexico-grammatical»
series of words, or as «lexico-grammatical categories.

It
should be noted that the term «part of speech» is purely
traditional and
conventional, it cannot be taken as in any way defining or
explana­tory. This name was introduced in the grammatical
teaching of Ancient Greece, where the concept of the sentence was not
yet explicitly identi­fied
in distinction to the general idea of speech, and where,
consequently, no
strict differentiation was drawn between the word as a vocabulary
unit and the word as a functional element of the sentence.

In
modern linguistics, parts of speech are discriminated on the basis of
the three
criteria:
«semantic»,
«formal», and «functional».

The
se­mantic
criterion
presupposes the evaluation of the generalized meaning, which
is characteristic of all the subsets of words constituting a given
part of speech. This meaning is understood as the «categorial
meaning of
the part of speech».

The
formal criterion
provides for the exposition of the
specific inflexional and derivational (word-building) features of
allthe
lexemic subsets of a part of speech.

The
functional criterion
concerns the
syntactic role of words in the sentence typical of a part of speech.
The said
three factors of categorial characterization of words are
conven­tionally
referred to as, respectively, «meaning», «form»,
and «function».

In accord
with the described criteria, words on the upper level of
classification
are divided into notional
and functional,
which reflects their division
in the earlier grammatical tradition into changeable and
un­changeable.

To the
notional
parts of speech
of the English language belong the noun, the adjective, the numeral,
the pronoun, the verb, the adverb.

Contrasted against the
notional parts of speech are words of incom­plete nominative
meaning and non-self-dependent, mediatory functions in the sentence.
These are functional parts of speech.

To
the basic functional
series of words
in English belong the article, the
preposition, the conjunction, the particle, the modal word, the
inter­jection.

Each part of speech after its
identification is further subdivided into subseries in accord with
various particular semantico-functional and formal features of the
constituent words. This subdivision is some­times called
«subcategorization» of parts of speech.

Thus, nouns
are subcategorized into proper and common, ani­mate and
inanimate, countable and uncountable, concrete and ab­stract,
etc.

Verbs
are subcategorized into fully predicative and partially pred­icative,
transitive and intransitive, actional and statal, purely nomi­native
and evaluative, etc.

Adjectives
are subcategorized into qualitative and relative, of con­stant
feature and temporary feature (the latter are referred to as
«statives» and identified by some scholars as a separate
part of speech un­der the heading of «category of state»),
factual and evaluative, etc.

The adverb,
the numeral, the pronoun are also subject to the cor­responding
subcategorizations.

We have
drawn a general outline of the division of the lexicon into part
of speech classes developed by modern linguists on the lines of
tra­ditional
morphology.

Alongside
the three-criteria principle of dividing the words into grammatical
(lexico-grammatical) classes, modern linguistics has de­veloped
another, narrower principle
of word-class identification based on syntactic featuring of words
only.

The fact is
that the three-criteria principle faces a special difficulty in
determining the part of speech status of such lexemes as have
mor­phological characteristics of notional words, but are
essentially distin­guished from notional words by their playing
the role of grammatical mediators in phrases and sentences. Here
belong, for instance, modal verbs
together with their equivalents — suppletive fillers, auxiliary
verbs, aspective
verbs, intensifying adverbs, determiner pronouns. This diffi­culty,
consisting in the intersection of heterogeneous properties in the
established
word-classes, can evidently be overcome by recognizing only one
criterion of the three as decisive.

Comparing
the syntactico-distributional classification of words with the
traditional part of speech division of words, one cannot but see the
similarity of the general schemes of the two: the opposition of
notional and
functional words, the four absolutely cardinal classes of notional
words (since numerals and pronouns have no positional functions of
their
own and serve as pro-nounal and pro-adjectival elements), the
in­terpretation of functional words as syntactic mediators and
their formal representation
by the list.

However,
under these unquestionable traits of similarity are distinctly
revealed
essential features of difference, the proper evaluation of which
allows us to make some important generalizations about the structure
of the
lexemic system of language.

As
a result of the undertaken analysis
we have obtained a founda­tion
for dividing the whole of the lexicon at the upper level of
classifica­tion
into three unequal parts.

The
first part of the lexicon forming an open set includes an
indefi­nitely large number of notional words which have a
complete nomina­tive
function. In accord with the said function, these words can be re
ferred to as «names»: nouns as substance names, verbs as
process names, adjectives
as primary property names and adverbs as secondary proper­ty
names. The whole notional set is represented by the four-stage
deriva­tional
paradigm of nomination.

The
second part of the lexicon forming a closed set includes substi­tutes
of names (pro-names). Here belong pronouns, and also broad-mean­ing
notional words which constitute various marginal subsets.

The
third part of the lexicon also forming a closed set includes
spec­ifiers
of names. These are function-categorial words of various
servo-status.

Substitutes
of names (pro-names) and specifiers of names, while stand­ing
with the names in nominative correlation as elements of the lexicon,
at
the same time serve as connecting links between the names within the
lexicon
and their actual uses in the sentences of living speech.

ЛИТЕРАТУРА:

1. Блох, М.Я.
   Теоретическая грамматика английского
   языка : Учеб. / М.Я. Блох. – 5–е изд.,
   стер. – М. : Высш. шк., 2006. – 423 с.

2. Блох, М.Я.
   Теоретические основы грамматики :
   учеб. / М.Я. Блох. – 3–е изд., испр. –
   М. : Высш. шк., 2002. – 160 с.

3. Blokh,
   M.Y. A course in theoretical English grammar / M.Y. Blokh. –
   M., 1983.

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In English grammar, a word class is a set of words that display the same formal properties, especially their inflections and distribution. The term «word class» is similar to the more traditional term, part of speech. It is also variously called grammatical category, lexical category, and syntactic category (although these terms are not wholly or universally synonymous).

The two major families of word classes are lexical (or open or form) classes (nouns, verbs, adjectives, adverbs) and function (or closed or structure) classes (determiners, particles, prepositions, and others).

Examples and Observations

• «When linguists began to look closely at English grammatical structure in the 1940s and 1950s, they encountered so many problems of identification and definition that the term part of speech soon fell out of favor, word class being introduced instead. Word classes are equivalent to parts of speech, but defined according to strict linguistic criteria.» (David Crystal, The Cambridge Encyclopedia of the English Language, 2nd ed. Cambridge University Press, 2003)
• «There is no single correct way of analyzing words into word classes…Grammarians disagree about the boundaries between the word classes (see gradience), and it is not always clear whether to lump subcategories together or to split them. For example, in some grammars…pronouns are classed as nouns, whereas in other frameworks…they are treated as a separate word class.» (Bas Aarts, Sylvia Chalker, Edmund Weiner, The Oxford Dictionary of English Grammar, 2nd ed. Oxford University Press, 2014)

Form Classes and Structure Classes

«[The] distinction between lexical and grammatical meaning determines the first division in our classification: form-class words and structure-class words. In general, the form classes provide the primary lexical content; the structure classes explain the grammatical or structural relationship. Think of the form-class words as the bricks of the language and the structure words as the mortar that holds them together.»

The form classes also known as content words or open classes include:

• Nouns
• Verbs
• Adjectives
• Adverbs

The structure classes, also known as function words or closed classes, include:

• Determiners
• Pronouns
• Auxiliaries
• Conjunctions
• Qualifiers
• Interrogatives
• Prepositions
• Expletives
• Particles

«Probably the most striking difference between the form classes and the structure classes is characterized by their numbers. Of the half million or more words in our language, the structure words—with some notable exceptions—can be counted in the hundreds. The form classes, however, are large, open classes; new nouns and verbs and adjectives and adverbs regularly enter the language as new technology and new ideas require them.» (Martha Kolln and Robert Funk, Understanding English Grammar. Allyn and Bacon, 1998)

One Word, Multiple Classes

«Items may belong to more than one class. In most instances, we can only assign a word to a word class when we encounter it in context. Looks is a verb in ‘It looks good,’ but a noun in ‘She has good looks‘; that is a conjunction in ‘I know that they are abroad,’ but a pronoun in ‘I know that‘ and a determiner in ‘I know that man’; one is a generic pronoun in ‘One must be careful not to offend them,’ but a numeral in ‘Give me one good reason.'» (Sidney Greenbaum, Oxford English Grammar. Oxford University Press, 1996)

Suffixes as Signals

«We recognize the class of a word by its use in context. Some words have suffixes (endings added to words to form new words) that help to signal the class they belong to. These suffixes are not necessarily sufficient in themselves to identify the class of a word. For example, -ly is a typical suffix for adverbs (slowly, proudly), but we also find this suffix in adjectives: cowardly, homely, manly. And we can sometimes convert words from one class to another even though they have suffixes that are typical of their original class: an engineer, to engineer; a negative response, a negative.» (Sidney Greenbaum and Gerald Nelson, An Introduction to English Grammar, 3rd ed. Pearson, 2009)

A Matter of Degree

«[N]ot all the members of a class will necessarily have all the identifying properties. Membership in a particular class is really a matter of degree. In this regard, grammar is not so different from the real world. There are prototypical sports like ‘football’ and not so sporty sports like ‘darts.’ There are exemplary mammals like ‘dogs’ and freakish ones like the ‘platypus.’ Similarly, there are good examples of verbs like watch and lousy examples like beware; exemplary nouns like chair that display all the features of a typical noun and some not so good ones like Kenny.» (Kersti Börjars and Kate Burridge, Introducing English Grammar, 2nd ed. Hodder, 2010)