Vibrato is a rapid, pulsating alternation between two very close pitches by a human voice or an instrument that is used to add expression to music. It is usually characterized in terms of both the amount, generally a semitone or less, and the speed of the pitch variation. Although the two terms are sometimes used interchangeably, vibrato differs from tremolo, which is a fast repetition of the same pitch. Vibrato is notated by a wavy horizontal line above the staff beginning over the note to which it is being applied and extending rightwards for its duration.