EXPLANATION Section 15.5 explains that apposition occurs when we place a second noun beside another one in order to define the first noun (for example Snoopy, the cartoon character, ...). For such apposition, we place commas before and after the noun placed in apposition. Another example would be:
Sometimes, however, words that usually function as nouns can be used as adjectives, as in sentences such as:
In that sentence, the word actor is actually used as a defining adjective and so we do not place commas around it. It becomes even easier to see if we leave out the article the:
Other examples where nouns function as adjectives are: seafood restaurant, telephone directory, computer malfunction and birthday party. When a noun is used as if it were an adjective, as in the second of each of the following pairs of sentences, commas are not used.
- Apposition: Television, a word made up from Greek elements, means seeing from afar.
- Adjective: The word television is made up from the Greek elements tele, meaning afar, and video-visus, meaning to see.
- Apposition: Tom Sawyer, a novel by Mark Twain, was written in 1876.
- Adjective: The novel Tom Sawyer was written by Mark Twain in 1876.
(Grammarians are actually divided about how to describe what is going on when nouns take on an adjectival function. Some prefer to still regard the noun as a noun rather than an adjective and to talk of restrictive apposition or defining apposition. Either way, the consensus is the same: commas should not be used.)