On a brass instrument, a valve is a mechanism that, when in use, directs the air column along additional tubing inside the instrument, thus producing a different fundamental and harmonic series.

Most modern valved instruments employ similar valve systems which results in the same patterns of valve combinations to alter a pitch. For example, depressing the first valve will normally result in lowering the original pitch by one step, or the interval of a second. The table below shows the valve combinations of a typical valved instrument and the resulting pitch alterations. It should be noted that the addition of the fourth valve became common in the mid-20th century to help produce better intonation with the third valve combinations that tended to be "out of tune" or having poor intonation (especially the 1st & 3rd valve combination). Valves can be created in a rotary or piston configuration.

See also rotary valve, piston valve.

Valve Combinations
Pitch Lowered
2nd Valve 1/2 step
1st Valve 1 full step
3rd Valve 1 & 1/2 steps
1st & 2nd Valves 1 & 1/2 steps
2nd & 3rd Valves 2 full steps
1st & 3rd Valves 2 & 1/2 steps
1st, 2nd, & 3rd Valves 3 full steps
4th Valve Combinations  
4th Valve 2 & 1/2 steps
2nd, & 4th Valves 3 full steps
1st, & 4th Valves 3 & 1/2 steps
3rd, & 4th Valves 4 full steps
1st, 2nd,, & 4th Valves 4 full steps
2nd, 3rd, & 4th Valves 4 & 1/2 steps
1st, 3rd, & 4th Valves 5 full steps
1st, 2nd, 3rd, & 4th Valves 5 & 1/2 steps

Animation of the trumpet valves.

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