Valves in a trumpet enable the player to lower, momentarily, the pitch of the note he wishes to make. What happens when one of the valves is pushed down is that the air is diverted through a small loop of tube thus lowering the pitch or sound of the instrument.
When the first valve is pressed usually by the first finger, the pitch of the trumpet will be lowered by a whole tone. The second valve, according to the same principle, lowers the sound by a semitone, and the third lowers it by a minor third.
Nearly all trumpets and brass instruments today are made with valves. The mechanism as invented in 1815 by two Germans. Today most valves are of the piston type with springs to return the valves to their original position.
The history of trumpets dates back to 1500 B.C in Egypt. But until the beginning of the 19th century all the variations produced by valves had to be made by the player controlling his breath.