Apostrophes’ chief purpose is to show possession, but these marks have other functions, too. They alert readers when, and where, one or more letters are missing from a word, such as the no that is dropped when cannot becomes can’t. Or they create separation to avoid confusion when two elements are combined for special reasons. For example, in When Colin writes a’s, they look like u’s, the apostrophes prevent us from thinking that the writer meant as and us. It’s hard to imagine any credible writer not using apostrophes for a’s and u’s, but beyond that, apostrophe unanimity is hard to find.

Apostrophes have a complicated relationship with plurals. Different writers have different approaches when writing the plural forms of abbreviations, some letters and numbers, and words that do not normally take plurals.

• What’s the plural of an abbreviation with periods, like Ph.D.? There’s no right answer. Some write Ph.D.s, some write Ph.D.’s.

• Most writers use apostrophes when pluralizing single capital letters (I earned three A’s), but there are some who would write three As. With groups of two or more capital letters, apostrophes seem less necessary (two new MPs, learn your ABCs), but some writers insist on them.

• Single-digit numbers are usually spelled out, but when they aren’t, you are just as likely to see 2s and 3s as 2’s and 3’s. With double digits and above, many (but not everyone) regard the apostrophe as superfluous. Most writers nowadays favor the 1900s, but some go with the 1900’s. If numerals are used to identify decades, the ’30s is widely used, but you will also see the 30’s, and occasionally even the ’30’s.

• A keyboard caveat: it takes extra effort to generate an apostrophe when it is the first character in numbers or words like the ’30s or ’tis (for “it is”). If you’re not careful, you’ll instead type an opening single quotation mark (‘), which is a backward and upside-down apostrophe. The result will be the ‘30s and ‘tis, which finicky readers consider an indefensible lapse.

• Making words plural with ’s is usually a big mistake, but some writers, as a courtesy to readers, will add ’s to words that don’t ordinarily become plural, as in no if’s, and’s, or but’s, or here are some do’s and don’t’s. Since two apostrophes in one word look clunky, you are more likely to see do’s and don’ts, which looks better, although don’ts is inconsistent with do’s. A better option might be to use italics to establish differentiation: no ifs, ands, or buts; some dos and don’ts.

• Let’s close with a possessive-apostrophe principle that confuses a lot of people. For the plurals of familiar compound nouns like driver’s license and master’s degree, the apostrophe stays the same; the plurals are driver’s licenses and master’s degrees. You may ask why not drivers’ licenses—after all, we’re talking about more than one driver, aren’t we? Well, yes and no. The driver’s in two driver’s licenses denotes that each license was issued to one driver only. The same reasoning applies to master’s degrees.

No punctuation mark causes more confusion and dissent than apostrophes. If we could get together on the rules, maybe people would use them more.