The diatonic harmonica has ten holes in the front of the comb, numbered 1 through 10 on the top reed plate, with 1 being the lowest-pitched notes and 10 being the highest notes. The comb itself is usually made of plastic or wood, although in some cases it could be made of metal. Behind the holes in the comb are the chambers, which direct air over the reeds. When you blow into a hole, the air passes through the chamber over the blow reed, causing it to vibrate, and out the slot. When you draw on a hole, the air enters through the slot and passes over the draw reed, causing it to vibrate, and exits out the chamber and hole.