Step 3: How the Harmonica Works
That's why playing the harmonica involves a combination of inhaling and exhaling (also called blowing and drawing). If you blow into hole 4 on a C harmonica, you'll hear a C. Now inhale–—the note moves up to a D. Blow into the next hole and it's an E. Inhale, F. Blowing into hole 6 produces a G, and inhaling makes an A. At this point you can probably guess what come next if you blow into hole 7: a B, right? Wrong! If you blow into hole 7, you will hear a C. You have to inhale to produce the B. As you can see, there isn't a set pattern to the blows and draws of a harmonica.
The three main oddities are holes 2, 7, and 10. In holes 7-9, the pattern mentioned above is changed so inhaling moves the note down instead of up.
Sound confusing? It is. The diagram above should make things much clearer.