Errant Apostrophes

The rules are pretty simple: Use an apostrophe to indicate possession (except with possessive its) and contraction. Don't put apostrophes where they don't belong. Some writers don't like to see a simple s after a word ending in a vowel; they're compelled to use an apostrophe. But fight that urge. These examples are all correct:

1960s, 1970s
Four Billys in this class
The Williamses next door
Dos and don'ts
The couple said their "I-dos" at the altar.