Language always is in a slow transition where spelling, pronunciation, definitions, and – more glacially –the grammar itself, is concerned. The competing spellings of co-worker and coworker is among those words in our language now undergoing just such a change.

Traditionally, co-worker is considered correct. As the Associated Press Stylebook still notes, typically words that denote position or occupation require a hyphen. Such words include co-author, co-owner and co-star.

The Chicago Manual of Styles 16th edition, however, lists coworker as not requiring a hyphen, a change from its 15th edition which allowed for either spelling. From personal experience as an editor, I’ve increasingly noticed writers using coworker without a hyphen, suggesting the 16th edition’s influence on writing and publishing.

Bottom Line: Use the style that your publisher prefers – if writing a magazine or newspaper article, that likely means AP style; if writing a book, that likely means Chicago Manual – and be internally consistent by always using the same style throughout your manuscript.