The apostrophe may be the most misunderstood punctuation mark in English—there are even websites dedicated to recording 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.
This small mark has two primary uses, signifying either 1. omitted letters or 2. possession.
Some common English words can be combined into a contraction, such as isn’t (is not), don’t (do not), and you’re (you are). 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”.
Possessive apostrophes help us visualize relationships. Rather than say “the dog that belongs to Sam,” one can add an ’s to the possessor (in this case, Sam) and say “Sam’s dog.” If the singular possessor ends with an s, add just an apostrophe instead of adding ’s—so, the dog that belongs to Jonas is Jonas’ dog and formulas that are used for mathematics are mathematics’ formulas.
If there are multiple possessors, as in “the teachers’ room” (which refers to a room that belongs to multiple teachers), the apostrophe is also placed outside of the s. But for plural nouns that don’t end in s, add the ’s, e.g. women’s rights.
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 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 quick trick: try replacing the ’re or ’s with are or is. If the syntax works, then you need the apostrophe; if not, it’s a possessive pronoun. For example, “It’s Sunday” can be written “It is Sunday,” while “The school locked its doors” can’t 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.”
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 in written pronunciation slang 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.