Two nouns referring to the same idea are often placed next to each other, sometimes with bracket-like commas around the second
DEFINITION AND GENERAL PURPOSE OF APPOSITION
Two nouns (or noun phrases) placed directly next to each other can do very different things. The first might resemble an adjective describing the second (see 38. Nouns Used Like Adjectives), or make a list with it (see 50. Right and Wrong Comma Places), or name an owner of what it represents (see 58. Optional Apostrophe Endings). In the kind of noun pair that I want to examine here, both of the nouns have similar meaning, like this:
(a) Two main ethnic groups populate Sri Lanka, an island south of India.
Here there are two noun phrases referring to the same island. They are different from the similar-meaning nouns discussed in the post 5. Repetition with Synonyms in that they are not separated by other words, and they differ from the kind of pairs considered in the post 92. Complement-Showing “As” in that the verb (populate) has nothing to do with their being together (it does not behave like CALL or MAKE).
The kind of noun pairing shown in (a) – sometimes with separating commas, sometimes not – has the technical name of “apposition”. Its purpose is to give the reader or listener a better idea of what the nouns mean. It usually involves just two nouns or noun phrases, but more than two are possible. It can be classified into different types according to the kinds of noun phrases involved. This post is about these various types of apposition, as well as their punctuation.
One form of apposition that I am not considering here, though, is where the second noun stands for part of the idea of the first, whether as a random example (after for example, such as or like) or as the part most deserving of a mention (with particularly, notably or especially). More about example-giving apposition is in the posts 1. Simple Example-Giving, 53. “As”, “Like” and “Such As”, 54. Sentence Lists 1: Incidental and 96. Hedging 2: Lists & Predictions.
THE MAIN TYPES OF APPOSITION
Apposition structures can be classified according to the kind of information that they give. They may be clarifying the meaning of the first noun, or identifying it more specifically, or presenting an alternative name for it. Most of these purposes are types of “good” repetition (see 24. Good and Bad Repetition). They can also be achieved by placing whole sentences together (see 18. Relations Between Sentences).
1. The Clarification Use
The following sentences all have this use:
(a) Two main ethnic groups populate Sri Lanka, an island south of India.
(b) Mount Kilimanjaro, an extinct volcano, is the highest in Africa.
(c) The defendant, a man of 45, was led in by two policemen.
(d) Ringo Starr, the Beatle drummer, was the last to join the group.
(e) Apposition, the juxtaposition of two or more nouns referring to the same thing, is a notable feature of professional writing.
(f) Felis leo, (in other words) the lion, is the acknowledged “king” of the cat family.
These illustrate different kinds of clarification. Sentences (a) – (d) may be called “characterising”, since the second noun phrase in the apposition gives a key characteristic of the idea expressed by the first. The first noun phrase may or may not be a “proper” noun with a capital letter (for different possible types, see 47. Article Errors with Proper Nouns). Sentence (e), on the other hand, is “defining”, since it clarifies the meaning of a technical term, whereas (f) is “synonym-giving” – linking a technical name with a more familiar everyday one. It is similar to, but not the same as, the name-presenting use illustrated by sentence (m) below. Note that only (e) and (f) can have in other words or that is to say before the second noun phrase.
In all of the above sentences, the second noun phrases are between two bracket-like commas (or a comma and a full stop). This use of commas is considered in detail in the post 50. Right & Wrong Comma Places. There is perhaps just one situation where it does not accompany apposition used for clarifying. Consider this:
(g) Paris the capital of France has a long history.
Unlike most of the first nouns above, Paris is a name with more than one owner – the other being a town in Texas, USA. Because of this, there will sometimes be a need to clarify which Paris is being talked about. This is what absent commas do in sentences like (g). The same thing happens in John the Baptist, Jack the Ripper and Ford the motor manufacturer. The presence of commas in (g) would suggest that the reader was expected to know already which Paris was meant, and that the second part of the apposition named a characteristic of that Paris that might have been forgotten or need highlighting.
This choice concerning commas in sentences like (g) is identical to the one in sentences containing who, which or that (see 34. Relative Pronouns and Commas). This is not surprising, since a paraphrase with a relative pronoun is usually possible: the Paris that is the capital of France in (g). More can be read about such sentences in the post 47. Article Errors with Proper Nouns.
2. The Identification Use
In this use, the second noun phrase in the apposition identifies or specifies who or what exactly is meant by the first:
(h) The upmarket cars Mercedes and Rolls Royce have iconic radiator designs.
(i) (The) Beatle John Lennon was killed in 1980.
(j) The article “a” is not possible with many nouns.
(k) An article, (namely) “the”, is usually needed with superlative adjectives.
(l) The Beatle drummer(,) (namely) Ringo Starr(,) was the last to join the group.
(m) The definite article(,) (namely) the(,) is not common before a generalising plural noun.
It will be seen that the first noun phrase in each of these is a description, while the second – the identification – is either a name or a more precise characterization. More on descriptions and names is in the post 62. Choices with Capital Letters. The identification in the first sentence (Mercedes and Rolls Royce) is also a list. More about lists in apposition constructions is in the post 54. Sentence Lists 1: Incidental. Identifications that are not lists are considered in detail in the post 117. Saying More Precisely What You Mean. Apposition is one of three main ways of expressing them.
Also noticeable above is that commas can be left out of all the sentences except (k). The punctuation depends primarily on whether or not the description expressed by the first noun phrase belongs to more than what the second noun phrase represents. It does in the first four sentences and it does not in the last two: popular cars, Beatles and article cover other possibilities besides the ones mentioned after them, but Beatle drummer and definite article do not.
The rule seems to be as follows. In multi-possibility sentences like (h)-(k) there is no choice about the punctuation: no commas when the first noun has the – even the omissible the in (i) – and commas when it has a(n) or no article at all. However, in single-possibility sentences like (l) and (m), there is a choice: no commas if the reader is expected to be familiar with the idea shown by the second noun phrase, and commas otherwise. In other words, commas are more suggestive of information-giving. Note that any presence of commas also makes it possible to add namely before the second part of the apposition.
It is possible to leave the unmentioned, as in (i), when two conditions are met. The first noun (which does not have to be multi-possibility) must represent a person (here Beatle), and it must not be followed by a comma. These conditions are also met by (l) when it is written without commas. Dropping the is usually a little informal and is very common in newspapers.
3. The Name-Presenting Use
Sometimes the second noun phrase in an apposition construction does not clarify or identify the meaning of the first one in any way, but is instead merely a different name for it that is not expected to be familiar to the reader. Here is an example:
(n) The shoulder blades, (in other words) the scapulae, are in a posterior position just below each shoulder.
A major reason why technical names like scapulae exist alongside everyday ones like shoulder blades is that scientific study has found a problem with the everyday name that is avoided by creating the technical one and defining it in a particular way. A probable reason for giving both names in sentences like (n) is in order to teach the technical name to the reader, who is perhaps expected to be a student. Starting with a familiar name before linking it with a new one is a classic way to assist understanding and memorization.
The punctuation of this apposition type is always two commas. Other possible linking expressions between the nouns are that is to say and or (without a following the).