Metronome mark
DEFINITION
Written at the start of a score, and at any significant change of tempo, this symbol precisely defines the tempo of the music by assigning absolute durations to all note values within the score. In this particular example, the performer is told that 120 crotchets, or quarter notes, fit into one minute of time. Many publishers precede the marking with letters "M.M.", referring to Maelzel's Metronome.