Valving, half valving and windsavers...

Each harmonica reed is mounted over a slot in the reedplate, cut to allow the reed to swing freely in and out of the slot.

When you have a hole with a blow reed mounted on the inside and a draw reed mounted on the outside, air can leak through the draw reed when you blow, and through the blow reed when you draw.

Bending on diatonics depends on the airflow affecting both reeds, as both reeds vibrate to create the bent note.

But having air go through both reed slots when only one reed is sounding is also leakage that can weaken the sound and use up you breath.

Chromatic harmonicas, and some diatonics, solve this problem by mounting a little flap of plastic (though leather has also been used in the past) over the slot, on the opposite side of the reedplate from the reed. These flaps are known as valves, windsavers, or windsaver valves.

Let's say you have a blow reed inside the slot, and also the slot for the draw reed, which is mounted on the outside. You mount a valve on the draw reed slot, also on the inside. When you play a blow note, your breath presses the inside valve against the draw reed slot, closing it off and directing all the air flow to the blow reed.

On the outside of the blow reed slot, the outside valve will get pulled against the blow reed slot when you inhale, again sealing ite blow slot off and directing all the inhaled air through the draw reed.

A fully valved harmonica valves both the blow and the draw slots. Most chromatics are valved up through Hole 10, sometimes higher.

A half-valved harmonica is one where only half the reed are valved. This is done so that you still have diatonic-style bends on the notes that usually bend, but you have louder notes on the non-bending ones, and those ones also bend, though not with the same sound or ease and the usual bends.