A hyphen is a short line (-) that connects words or parts of words. You see hyphens in words like self-control, twenty-one, and mother-in-law. The hyphen shows that these separate words work together as a single idea.

Hyphens help prevent confusion. Without the hyphen, “a little used car” could mean either a small car that's been used, or a used car of any size. But “a little-used car” clearly means a car that hasn't been driven much. The hyphen joins “little” and “used” so they work together to describe the car.

You'll also find hyphens when words break across lines in books. If “wonderful” doesn't fit at the end of a line, it might split as “won-” on one line and “derful” on the next. The hyphen shows the word continues.

Some compound words start with hyphens (re-examine), keep them permanently (merry-go-round), or eventually drop them as they become more common. Email, for instance, used to be written as e-mail. The rules can seem arbitrary, which is why even professional writers keep dictionaries handy. When you're unsure whether a compound word needs a hyphen, looking it up is always smart.