Water key

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, less euphemistically, a spit valve. In valved instruments like trumpets, they are found always on the main tuning slide, and sometimes 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 at the bottom of the loop at the top of the instrument.
Water valves are often erroneously referred to as "spit valves", as the myth persists that some or much of the liquid consists of spit. Rather, it consists of condensed moisture from the breath of the player. The amount of water accumulated is directly in proportion to the size of the instrument, and the amount of metal exposed to the air, which in turn enables the process of condensation due to warm, moist air from the lungs meeting metal cooled by room-temperature air.
History.
Historical instruments, like natural trumpets or natural horns, did not have water keys. The player would rotate the instrument in order to expel the fluid either through the bell or through the mouthpipe. On older valved instruments (as well as on some modern instruments), it may be necessary to pull out one or more valve slides in order to completely empty the instrument. Turning the instrument upside down and actuating the valves may also release fluid stuck in the valve slides into the main tubing of the instrument, where it can then be emptied by turning the instrument or by using a water key on the main tubing.
Some trumpet designs, notably Bach Stradivarius trumpets in B♭, lack a water key on the third valve slide. Instead, they have a "dump slide", a small slide inserted into the third slide. When the dump slide is pulled out, any fluid in the third valve slide will come out.
Designs.
Water keys are made in various designs. All have the same purpose, to empty the condensation liquid from an instrument without having to rotate the instrument or pull out the slides.
The traditional design features a simple lever key as found on woodwinds, but with a cork rather than a pad in the cup. 
A spring holds the cork against a raised hollow cylinder mounted on the slide or loop during play. To drain, the player presses the non-cup end of the key towards the instrument to open the valve.
Another design is called an Amado water key after its inventor. It consists of a short hollow cylinder mounted transversely on the slide. The cylinder has a button on one side that operates an enclosed stopper valve held shut with a spring, and a hole on the bottom side to drain the water. The player presses the button and blows into the horn to drain the water. The Amado design has the advantage that it presents less of a deviation from a smooth inner slide wall compared to the larger-volume drain port in the traditional design. Nevertheless, most modern horns are still fitted with the traditional design of water key.
The Saturn key, invented by Denis Wedgwood and sold by Wedgewood Brass, aimed to improve on the Amado key by offering an easier evacuation when open. The valve takes the shape of a ball with a ring around the waist, like the planet Saturn, hence the name. The mechanism contains a small steel ball, held in the ring, which blocks the hole. By pushing the ring in any direction, the player moves the ball out of the way and the fluid can be ejected. The ring is spring-loaded and returns the ball to its seating when released.