Most harmonicas have ten holes and twenty reeds. Each hole has two because a different one is active when you blow or draw through the hole. Why are they called that? Harmony, idiot. Harmonicas come in a particular musical key, and not all notes are present. (ok, they are if you know how to bend) While this is sometimes an inconvenience, the nice part is that if you accidentally (or intentionally) blow/suck through the hole next to the target hole you were going for, it doesn't sound as bad as it would on, say, a piano. In fact, sometimes it even sounds good. That's called a chord.