A mechanical or electronic device that keeps a regular beat and may be adjusted to any desired speed ( tempo), used by musicians for practicing difficult rhythmic passages.The metronome was invented and patented by the German inventor, Johann Nepomuk M'lzel (August 15, 1772 - July 21, 1838) in 1816, on the suggestion of composer Ludwig van Beethoven. The marking M.M. at the beginning of a composition originally stood for " M'lzel M etronome," but has since come to designate " Metronome M arking." This marking identifies the tempo of the composition in terms of the number of beats per minute. Thus, if the M.M. shows the quarter note to equal 60, that means the tempo should be 60 beats per minute or one beat per second. See also M.M. ; metronome marking.<br><br>A clicking electronic device that sets tempo for a rhythm or song. It measures time in BPM (beats per minute). A popular "middle of the road" metronome setting is, quarter note = 120 bpm.