“Backward” and “backwards” can both be adjectives and adverbs, so you can say, “Squiggly jumped backward when Aardvark shouted, ‘Boo!’”—that’s using “backward” as an adverb. It’s describing the way Squiggly is moving, the same way you might say he jumped frantically or jumped high.
And you can say, “Grammar Girl wishes her Xbox had backward compatibility,”—that’s using “backward” as an adjective. It’s describing the type of compatibility I wish I had, the same way you might say a fabulous couple has perfect compatibility.
Adjectives: ‘Backward’ and ‘backwards’
As an adjective — in that “backward compatibility” way — Garner’s Modern English Usage says only “backward” is allowed. And a Google Ngram search, which shows how often words are used in the books included in the Google Books database, shows that “backwards” as an adjective is almost nonexistent in both American English and British English.
I do have to say I’ve seen it quite a bit online, as in “backwards compatibility,” but that’s online. If you want to write properly for your job or for schoolwork or just for life, stick with “backward” for the adjective.
Adverbs: ‘Backward’ and ‘backwards’
When we get to the adverb, it’s a different story. Both “backward” and “backwards” are correct. You can say either “Squiggly jumped backward” or “Squiggly jumped backwards.” The big difference is between British English and American English, although it’s not as stark as some differences I’ve seen.
Also, I can find both spellings on the British BBC website, but “backwards” seems far more common, as in this headline: Ex-soldier Wayne Ingram walks 70 miles backwards for spinal charity. Good job, Wayne! And I can find “backwards” on the New York Times website, but “backward” is far more common.
So “backward” is the more American choice, and “backwards” is the more British choice, but you will find both in both languages.
The way I remember that “backward” is the word in American English, is to remind myself that Americans like shortcuts. For example, I’m willing to bet we eat in our cars more often than British people do. So think about how Americans like shortcuts, and then think about how we lopped the S off “backwards” to make it shorter.
If you choose to use “backwards” as an adverb in the United States, it’s not wrong, but it may look a little weird to people. It’s like spelling “colour” with a U; it draws attention to itself and, I suppose, could potentially be distracting to American readers.
This story has been updated from an earlier version posted on November 23, 2011.