How can a bugle play a tune with no valves or keys?

A bugle plays tunes by jumping between the natural harmonics of its tube. Any column of air, like any string, vibrates at many frequencies at once: a lowest one, the fundamental, plus higher ones at exactly two, three, four, five times that frequency. That ladder of whole-number multiples is the harmonic series. A bugle has no valves, so it cannot change the length of its tube; instead the bugler changes the air and the tension of the lips, and the tube 'locks on' to a different rung of the ladder.

The rungs are musical. The second harmonic sounds an octave above the fundamental, the third a fifth above that, the fourth two octaves above the fundamental. A bugle normally uses only the second to sixth harmonics, which happen to spell a major chord, for instance B♭, D and F. That is why every bugle call, from reveille to Taps, sounds like a broken chord: by definition, a bugle call uses notes from a single overtone series and nothing else.

Soldiers lived by those few notes. Every duty in camp had its own call, and the cavalry, with horses to look after, heard twice as many signals as the infantry. Taps, built from the same handful of harmonics, still marks the end of the day and is sounded at military funerals.