Water key

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A water key is a valve or tap used to allow the drainage of accumulated fluid from wind instruments. It is otherwise known as a water valve or spit valve. They are most often located at a low bend, where gravity assists fluid collection. In valved instruments such as trumpets, cornets and flugelhorns they are placed under the lowest bend of the main tuning slide and on valve slides. In the trombone, it is on the lower side of the bend in the hand slide. Baritone saxophones have a water key attached below the top loop of the instrument.

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. Without such valves it would be necessary to upend the instrument to clear the tubing and sound path, to avoid popping that occurs when the player's breath bubbles through fluid that is blocking its path.

During inhalation, warm breath in the upper airways becomes saturated with water. In exhalation much of it is recovered, re-coating the mucous membranes. 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. Larger instruments collect condensate more efficiently; the amount of condensate accumulated is in direct proportion to the area of the instrument surface—the amount of metal exposure separating breath from ambient air. When warm moist air from the lungs makes contact with room-temperature metal, water droplets form as on a cold can of soda.