An appositive is an identifying word or phrase that follows a noun. These phrases provide the listener or reader with additional information about the preceding noun that the noun itself does not provide. Appositives exist in most languages, since every modern language has nouns, but the manner in which appositives fit into a sentence can vary depending on the language. In English, some appositives must be separated by commas, while others do not need extra punctuation.
Even though they modify nouns, appositives actually consist of nouns and noun phrases. These words and phrases are usually non-essential since the sentence can typically stand on its own without them. The purpose of an appositive is simply to provide the reader or listener with more extensive information. For example, in the sentence, "My sister's best friend, Mary, went to the doctor," the appositive is "Mary," and "best friend" is the noun phrase that "Mary" describes. The sentence makes complete sense even without the inclusion of an appositive, but adding the proper noun "Mary" gives the reader more information about the identity of the subject in question.
Within the English language, most appositives come after the nouns they identify. The same appositive can usually be placed before the identified term, however. While this practice is still grammatically correct, native English speakers rarely place appositives before the identified noun during casual conversation.
In the sentence, "George Washington, the first president of the United States of America, took his oath of office on April 30, 1789," "George Washington" is the subject noun and "the first president of the United States of America" is the appositive phrase. An individual could also write, "The first president of the United States of America, George Washington took his oath of office on April 30, 1789." The subject noun and appositive remain the same in both instances, and both sentences are technically grammatically correct. Most speakers would use the former phrasing, however, rather than the latter.
Appositives are almost always set apart from the rest of the sentence by commas, but some appositives do not need the extra punctuation. If the descriptive phrase provides unnecessary information, the writer must separate the phrase from the rest of the sentence using commas. On the other hand, if the descriptive phrase provides necessary information, no commas are needed.
For instance, in the sentence, "Billy played baseball with his friend Robert," "friend" is the noun being identified and "Robert" is the appositive. Assuming Billy has more than one friend, the writer must identify the friend in question in order to let the reader know which friend Billy played with. As a result, no comma is needed. If the sentence reads, "Billy played baseball with his best friend, Robert," the name of the friend is no longer vital information due to the fact that Billy only has one best friend. Thus, the reader already knows who the writer is referring to, even without the name, and the identifying noun needs to be offset with a comma.