Tremolo is a trembling or shivering effect that is produced by a very fast repetition of a single note or by a rapid variation in its loudness. This term is also sometimes used to mean vibrato, which is a very fast alternation between two very similar pitches. Tremolo is mostly performed on violins and other bowed string instruments, as it is easier to execute on them than on wind and other instruments. The most common type of tremolo today is unmeasured tremolo, in which a note is repeated as rapidly as possible rather than trying to play a specific number of repetitions per beat.
That a note is to be played as a tremolo is usually indicated by placing three parallel, slightly diagonal strokes through its stem, identical to the notation indicating that a note is to be played as a series of repeated demisemiquavers (thirty-second notes). If the note does not have a stem, such as with a semibreve (whole note), the three strokes float above the note.