A spell-checker is the piece of software that checks your spelling — the thing that underlines an unknown word in red and offers you a list of near-misses to choose from. Every word processor, email program and phone keyboard has one built in. The difference between spell-checker and spell-check is small but real: spell-checker names the tool, always a thing ("this spell-checker doesn't know British spellings"), while spell-check can be either the tool or the action, and can also be a verb. So spell-checker is the safer word when you mean the program itself, and you would not say "I spell-checkered it" — the verb is always spell-check. Spelling of the word itself varies: spell-checker and spellchecker both appear, hyphen optional. How it actually works is worth knowing, because it explains its limits. It holds a dictionary list and compares each string you type against that list. If your string is on the list, it says nothing. If it is not, it marks it. That means two kinds of trouble. It marks correct words it simply doesn't have — people's names, Taiwanese place names, new slang, technical terms — and those red lines are false alarms. And it stays quiet about wrong words that happen to exist: write "quite" for "quiet", or "manager" for "manger", and the spell-checker approves both. It also needs to be set to the right variety of English, or it will keep objecting to colour, or to color, depending which way it is pointed. Most spell-checkers let you add words to a personal list, which is what you should do with your own name instead of ignoring the underline forever.