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 where gravity assists the fluid collection, in such valved instruments such as trumpets, cornets and flugelhorns under the lowest bend of the main tuning slide and on valve slides. On 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", condensation of breath is the accumulated moisture which a water key drains, so upending the instrument to clear the tubing and sound path is not necessary, for when the level rises above the bend unwanted popping occurs as the sound of blowing bubbles through that fluid blockage—a drainage problem which waterkeys solve. Primarily condensed moisture, food particles, but very little actual spit collects from the breath of the player for as they inhale, the upper airways warm and saturate with water before entering the lungs, which is recovered mostly at exhale re-coating the mucus membranes. Saturation increases as lung pressure does while passing the embouchure, and, like a thermal expansion valve in a refrigerator or A/C, the sudden drop in pressure which drops temperature further assists the condensation of the player's warm moist breath. Larger instruments collect condensate more efficiently and the amount of condensate accumulated is directly in proportion to the area of the instrument surface—the amount of metal exposure separating breath from ambient air—which enhances the process of condensation as the warm moist air from the lungs forms droplets as it makes contact with room-temperature metal—such as seen on a cold can of soda.