162. Writing about Categories


English has a wide variety of ways of naming general categories and their members


Categories are groups of similar things. Recognising similarities and differences is thus an important part of making them. They are especially important in professional writing, on a par with examples, consequences, data sources and opinions.

Like these other topics, categories in English are associated with a wide range of vocabulary and grammatical structures. Some of these feature within this blog under other headings. However, bringing a more complete list of possibilities together in one place seems useful to do to facilitate comparison and highlight some interesting subtleties.

Category language can be divided into two main kinds: that for naming category members after first mentioning the category, and that for naming a category to which previously-mentioned items belong. Each of these is dealt with in turn below.



The language for this kind of naming varies according to whether it names all of the category members or just some of them. Compare:

(a) (NAMING SOME) Countries with a cool temperate climate include New Zealand (and Canada).

(b) (NAMING ALL) Animals comprise vertebrates and invertebrates.

The meanings of “all” and “some” are expressed by the underlined verbs. Note the absence of of after COMPRISE (see 42. Unnecessary Prepositions). The verb subjects (before them) are category names, the objects (after) are category members.

After INCLUDE, one or more category members may be mentioned, but not all of them. This means nouns after INCLUDE are usually examples (see 1. Simple Example-Giving). After COMPRISE, it is logically necessary to name at least two category members, creating what I have elsewhere called a “sentence list” (see 55. Sentence Lists 2: Main-Message).

The above verbs are not the only language for naming category members. Alternatives are as follows.


1. Alternatives to INCLUDE

A common alternative is to combine a starting number word like one with the verb BE. However, there is a difference depending on the kind of category member being named. Compare:

(c) One country with a cool temperate climate is New Zealand.

(d) One (sub)group of animals is vertebrates.

In (c), the category member New Zealand is an individual – there is only one. In (d), however, the category member vertebrates is itself a category containing numerous members. As a result of this difference (c) has one by itself before the category name, whereas (d) has one (sub)group of (i.e. not *one animal). An alternative to one in both cases is a (for the difference, see 67. Numbers in Spoken English).

Instead of one in (c), you can also say one example of a. In (d) there are various substitutes for (sub)group, such as branch, category, class, division, group, kind, sort and type. The underlined ones can similarly have the prefix sub-.

Other number words become necessary if more than one category member is being named. For example, to mention Canada in (c) alongside New Zealand, you would start with two. You could also say some.


2. Alternatives to COMPRISE

A very common way of naming a full list of category members is by starting with there are and a number word:

(e) There are three primary colours: red, yellow and blue.

(f) There are two (sub)groups of animals: vertebrates and invertebrates.

Sentences like this are not at all informal (see 161. Presenting Information with “There”). If you are not sure about the accuracy of the number word, you can add a word meaning main after it (see 96. Hedging 2: Lists & Predictions). Note the need for a colon before the list of category members (see 55. Sentence Lists 2).

Once again, the wording after the number word depends on whether the category members are individuals or subgroups. Individuals usually require the class name to be mentioned immediately (primary colours in [e]), whereas subgroups need the extra words subgroups of (or similar), as in in (f).

An alternative to there are at the start of the sentence is the name of the category. This is when the verb can be COMPRISE, as in (b) above. However, other verbs are possible too. Before category members that are individuals you can simply say are:

(g) The (three) primary colours are red, yellow and blue.

For more on this way of using BE, see 117. Saying More Precisely What is Meant.

Before category members that are themselves categories, COMPRISE can be replaced by the passive form of verbs like BREAK, CATEGORISE, CLASSIFY, DIVIDE, GROUP, ORGANISE, SEPARATE, SPLIT and SORT. They usually need into after them – not in or to, which are common errors of writers not brought up speaking English. Here is sentence (b) rephrased with one such verb:

(h) Animals are divided into (two groups:) vertebrates and invertebrates.

The part in brackets illustrates another feature of sentences like (b) and (h), where the category members are themselves categories: you can add a number expression like two groups, plus a colon. Neither of these can stand alone: they must be both present or both absent. The reason why dropping two subgroups removes the need for the colon is that the preceding words then cease to be a possible complete sentence, thus failing to meet the conditions for a colon (see 17. Colons versus Semi-Colons).

When a number expression and colon are present before a subcategory, some active verb alternatives to COMPRISE also become possible. You can use fall into with classes, groups or divisions after the number word, and are of with types, sorts or kinds (e.g. animals are of two types).

There are some special benefits in using a passive classification verb in the way shown in (h). One is that writers can show whether or not the classification is their own. This is done by choosing between can be and are in front: can be says that the classification is the writer’s, while are shows it to be somebody else’s just being reported. For more on using can to show a personal perception, see 107. The Language of Opinions.

Another benefit of passive classification verbs is that they allow the writer to say how subcategories differ from each other. Consider this:

(i) Words were traditionally classified according to their meanings into eight “parts of speech”.

This says that meanings used to be how parts of speech were distinguished from each other – they were the “criterion” for the classification. According to is one common way of signalling a criterion; depending on, on the basis of or in terms of are also possible. All are examples of “multi-word” prepositions (see 84. Seven Things to Know about Prepositions).

The preposition nature of these phrases means that they have to be followed by a noun or equivalent (meanings above). Statements containing a verb can be converted into noun equivalents for this purpose, but not by the normal means of adding the fact that before them (see 153. Conjunction Uses of “that”). Instead, they need a question word (whether, how, where etc.) so that they become indirect questions (see 57. Indirect Questions in Formal Writing), e.g.:

(j) Words can be divided into various “word classes” according to how they are used.



The simplest way to name the category to which something belongs is with BE:

(k) Red is a primary colour.

(l) Mammals are vertebrates.

This is a different kind of BE from that in (g) above: it means “belong to” rather than “equate to”. Indeed, BELONG TO is a possible alternative, with or without a phrase like the class of. There is also a passive equivalent: BE INCLUDED IN. In both cases the subsequent category name must be plural (primary colours in [k]).

Another possibility is the passive form of CATEGORISE or CLASSIFY, which once again allows the special choice between are and can be. This meaning of these verbs rather than the earlier-described one after a category name – as in (j) – is signalled by a change of following preposition from into to as. Thus, instead of are in (l) we could say can be (or are) classified as. The same verbs could also be used in the active form (Scientists classify … as …), in which case the category name after as is an “object complement” (see 92. Complement-Showing “As”).

The idea of “belonging” can also be expressed with nouns. After single category members, one can say is/are a member/example of (+ plural). Example can also have the singular form of the category name, like this:

(m) Red is an example of a primary colour (primary colours).

Using example after a category member (red) that is not the main or “new” information in the sentence does not seem to be “true” example-giving (see 1. Simple Example-Giving).

After category members that are themselves categories, use are a … of (+ plural), inserting any of the synonyms of subgroup listed above, or say are members of the class of. Note that kind of, sort of and type of can go before singular as well as plural nouns, but without a (… a type of vertebrate) – a rare case of a singular countable noun used without an article or equivalent in front (see 110. Nouns without “the” or “a”).



To assist understanding and memorization of the vocabulary in this post, below are some sentences with blank spaces. Each of the spaces represents one of the words discussed above. The task is to identify the words, using the diagram to help you, and put them into the appropriate grammatical form (singular, plural, etc.). Alternative possibilities may sometimes exist. Suggested answers are given afterwards.


1. Cold-blooded vertebrates ….. reptiles and fish.

2. Mammals ….. to the ….. of warm-blooded vertebrates.

3. Crocodiles are a …… …… amphibian.

4. Warm-blooded vertebrates …… …… into mammals and birds.

5. Reptiles are …… of the …… of cold-blooded vertebrates.

6. Dolphins …… …… as mammals.

7. …… …… of fish …… sharks and tuna.

8. …… …… …… main …… of ……-……  ……: reptiles, fish and amphibians.

9. Warm-blooded animals …… …… …… …… vertebrates.

10. Vertebrates …… mammals, birds, ……, ……  and …… .


Possible Answers

1. Cold-blooded vertebrates INCLUDE reptiles and fish.

2. Mammals BELONG to the CLASS of warm-blooded vertebrates.

3. Crocodiles are a KIND/TYPE/SPORT OF amphibian.

4. Warm-blooded vertebrates ARE DIVIDED (etc.) into mammals and birds.

5. Reptiles are MEMBERS of the CLASS of cold-blooded vertebrates.

6. Dolphins ARE CLASSIFIED as mammals.

7. TWO SUBGROUPS of fish ARE sharks and tuna.

8. THERE ARE THREE main GROUPS of COLD-BLOODED VERTEBRATES: reptiles, fish and amphibians.

9. Warm-blooded animals ARE A SUBGROUP OF vertebrates.

10. Vertebrates COMPRISE mammals, birds, REPTILES, FISH and AMPHIBIANS.


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