The next time you write one of these words, you may want to back up and see if you’ve got it right. Knowing which part of speech you’re using will prove helpful.

“Backup” as a single a word with no space or hyphen can be used either as an adjective (When Johnson got knocked out, the coach put in his backup quarterback.) or as a noun (Noticing that the license plate matched that of a stolen vehicle, the policeman called for backup.).

“Back up” as two words with a space is a verb meaning to go in reverse (“Stop and back up, I think someone was in the alley,” the deputy said to his partner.).

Sometimes “back-up” with a hyphen appears as a replacement for “backup.” The hyphen is unnecessary, though, so back up and go with a single word.