Water key

A water key is a valve or tap used to allow the drainage of accumulated fluid from wind instruments.

While often referred to as "spit valves", this is a misnomer, as the fluid to be removed is not only saliva, but condensation of moisture from the player's breath.

The change from high to low pressure as the moist air passes through the embouchure leads, as with a thermal expansion valve in a refrigerator or A/C, to further condensation.

When warm moist air from the lungs makes contact with room-temperature metal, water droplets form as on a cold can of soda.

The traditional design features a simple lever key as found on woodwinds, but with a cork rather than a pad in the cup.

During normal play a spring presses the cork or pad tightly, preventing air leaks, against a raised hollow cylinder mounted under the slide or loop.

The player drains excess fluid before the tone becomes distorted by the accumulation of fluid, will open the water key by squeezing the lever end of the key then blow to speed the drain as rests allow.