In each chamber of a standard single reed diatonic harmonica, there are two reeds. During "normal" play, one of them responds to positive pressure (blowing) and one of them responds to negative pressure (drawing). However, the two reeds can be made to interact in very interesting (and useful) ways, the most well-known being the typical blues-style bend. In this, the higher pitched reed of the pair can be lowered in pitch to somewhere just above the lower pitched reed in the same hole. In the case of a draw bend, when the note is bent to its lowest pitch, most of the sound is coming from the blow reed, vibrating at slightly higher than its normal rate. In the case of a blow note being bent as far as possible, most of the sound is coming from the draw reed. This can be verified by playing the harp without covers and blocking the reeds with the fingertips. The reverse can also be made to happen, which results in an overblow or overdraw. In the case of the former, by blowing in a particular manner, the blow reed can be made to "stall" and the draw reed vibrate at a higher pitch. With overdraws, the draw reed is stalled and the blow reed is made to vibrate at a higher pitch. By use of this technique in addition to note bending, a complete chromatic scale can be played on the humble diatonic instrument. However, the challenge to the player is to make this all happen in a controlled and musical way - easier said than done.
