A lever used on certain brass instruments that provide special functionality. The lever will typically operate a rotary valve to add tubing to the instrument and lower the key (tonality) of the instrument's harmonic overtone series or control the movement of one the tuning slides to provide a high degree of control over intonation. The trigger is typically operated by the index finger, thumb, or fourth (little) finger depending upon the instrument. The most common use of the trigger is on trombone, horns, tubas, and trumpets.
Horn Thumb Triggers
The modern horn is an instrument in the key of F. The double horn has a trigger to put the horn into the key of B-flat (raising the key by a perfect fourth). A double horn will normally have two full sets of tubing (one set for F and one for B-flat) and the thumb trigger will route the air stream through the second set of tubes. The air stream is directed into the F tubes (F side) or the B-flat tubes (B-flat side) depending upon whether or not the trigger is engaged. The F side of the horn works well for the lower pitches and the B-flat side of the horn is better suited for the higher register of the instrument since the tubing is shorter.
The trombone is in the key of B-flat. Typically the bass trombones and many large bore tenor trombones will have a trigger to put the instrument into the key of F (lower the key by a perfect fourth). This allows the performer to perform the low C and B-flat (on the bass clef staff) in 1st and 2nd positions rather than the normal 6th and 7th positions providing better flexibility in fast passages. It also extends the playable notes in the low range between the F below the staff (FF) to the low C (CC). As a side note, the extra tubing associated with the trigger systems will always have a tuning slide to allow the performer to correctly tune that portion of the instrument. Often long tuning slides are used that can be pulled out far enough to allow the same attachment to be tuned to E (a half step lower) and increase the range of possiblilities for certain musical passages. The use of the trigger also changes the positions of the notes slightly.
Bass trombones often have a second trigger which is either "dependent" or "independent" of the first trigger. That means, in the dependent trigger system, the second trigger will not work unless the first trigger is also engaged. The independent trigger system allows each trigger to be used independently. Typically, the second trigger will be in the key of D (extendable to D-flat). The use of the independent trigger system will have four keys available in one trombone: B-flat, F, G (or G-flat), and D (or D-flat).
Trumpet & Tuba Triggers
Trumpets and tubas will occasionally have trigger devices that will allow the performer to slightly extend the first or third valve tuning slides to adjust the intonation for certain valve combinations. They are most commonly spring-loaded so the tuning slide (once extended) will return to it's original position once the trigger is released.