The word diatonic means that all the notes on the instrument belong to one scale or key. If you have a harmonica in C (the key is engraved onto the top of the cover on the right), the notes you can play all belong to the C major scale or the key of C. On the piano, this would be the equivalent of only playing on the white keys. Similarly, if you have a harmonica in G, you'll only be able to play notes which belong to the key of G.
You can buy diatonic harmonicas in many keys, which is necessary for a few reasons. If you play with other musicians, they'll be playing in many keys. Guitar players prefer the keys of C, G, D, A, and E; they are easier to play and sound better on that instrument. If you're playing in a band that has trumpets or saxophones, they'll probably prefer keys like F, B flat, E flat and A flat; these are the most congenial keys for those instruments.
This program uses the C diatonic harmonica to demonstrate the instrument.
The 10 holes of the diatonic harmonica can produce 20 notes. Each hole produces one note when you blow and a different note when you draw. Starting with the C below middle C on the piano, the blow notes on the C harmonica are: C E G C E G C E G C. The draw notes are: D G B D F A B D F A.
If you arrange these notes in ascending order, the result (starting with middle C, hole 4) is a two octave C major scale. In this way, the harmonica can be used to play any melody in the key of C as long as it doesn't use sharps or flats (explained in a later lesson). Later we'll show you that this limitation can be overcome by advanced techniques. For now, it's enough to know that the diatonic harmonica or harp is the one often heard in rock, folk, blues and country recordings.
