This can be a daunting subject, so don't worry too much if you don't understand everything on this page.

If you buy a diatonic harmonica in the key of "C" and then play blues on it, what your are actually doing is playing in the key of "G" on a "C" harmonica, this is know as playing in 2nd position, confused? Because the diatonic harmonica is not a chromatic instrument you have to play it in different "positions" to be able to play different scales and different tunes. As a general rule tunes and melodies, like "Oh When The Saint Go Marching In" are played in 1st position, "C" on a "C" harmonica, using blow hole 4 as your root, or key, note. Blues harmonica is played in 2nd position, using suck hole 2 as your root, or key, note. Minor tunes are played in 3rd position, using suck hole 4 as your root, or key, note. There are many other positions which means you can play in all 12 keys on one harmonica, but 1st, 2nd and 3rd position are the most common.

The chart below shows you what key harmonica to play in relation to what the accompaniment is in.

So when a guitarist, or any accompanist says "This is a blues in the key of "A", you would pick up your "D" harmonica for 2nd position.