Almost all brass instruments have air column resonances which are close to forming a harmonic series; this gives rise to the familiar pattern of “bugle call†natural notes. In the upper register the harmonics are close enough together to allow a diatonic scale to be played without modifying the tube length, although certain harmonics have to be modified in pitch by lipping or hand-stopping. The much larger pitch intervals between the lower harmonics can be filled in by changing the tube length: this can be done either by a slide as on the trombone, by a set of valves as on the trumpet, or by a set of finger holes as on the serpent.