Metronomes may be used by musicians when practising in order to maintain a constant tempo; by adjusting the metronome, facility can be achieved at varying tempo.
Tempo is almost always measured in beats per minute (BPM); metronomes can be set to variable tempi, usually ranging from 40 to 208 BPM; another marking denoting metronome tempi is M.M. (or MM), or Mälzel's Metronome. The notation M.M. is often followed by a numeric value indicating the tempo, as in M.M. = 60.