The apostrophe may be the most misunderstood punctuation mark in English. There are even websites dedicated to cataloging its misuse. Most punctuation marks fall between words to separate ideas or grammatical clauses, but the apostrophe is used within words and to combine multiple words which befuddles even native speakers.
This small mark has two primary uses: to signify either possession or omitted letters. Rather than say “the friend of Sam,” one can say “Sam’s friend” by adding a ‘s to the possessor (in this case, Sam). As for the second use, some common English words can be combined into a contraction, such as isn’t, don’t, and you’re. We often elide sounds and letters when speaking for the sake of convenience, and the apostrophe helps written language reflect its spoken equivalent. The word apostrophe comes from the Greek word apóstrophos which refers to a mark used in Greek to signify an omitted letter. It literally means a “mark of turning away.”
The apostrophe causes so much strife in part because it’s the culprit in two of the most commonly confused pairs in English: you’re/your and it’s/its. Possessive pronouns (like your and its) never take apostrophes, but their soundalike friends are contractions that require apostrophes. We all struggle with these when writing and proofreading our work, but here’s a trick: try replacing the ‘re or ‘s with are or is. If the syntax works, then you need the apostrophe; if not, it’s the possessive pronoun. For example, “It’s Sunday” can be written “It is Sunday,” while “The school locked its doors” cannot be written “The school locked it is doors.” Likewise, “You’re late” can be written “You are late,” while “I saw your note” cannot be written “I saw you are note.”
What about when you see an apostrophe at the end of the word? If there are multiple possessors, as in the teachers’ dog (which refers to a dog belonging to multiple teachers), the apostrophe is placed outside of the s. But in English there’s always an exception. For plural nouns that do not end in s, add ‘s, e.g. women’s rights. Also, the apostrophe is added for the possessive of a noun that is plural in form but singular in meaning, e.g. mathematics’ formulas. Lastly, for joint possession, the ‘s is added to the word nearest the object of possession, e.g. Francis and Kucera’s book.
The apostrophe has a number of other lesser-known uses. It can replace omitted numbers (e.g. the class of ’72, the ’20s, etc.) and letters e.g. gone fishin’. It can also be used to indicate plural letters, as in p’s and q’s, two A’s and four B’s, etc.
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