The first is a rapid reiteration:
- of a single note, particularly used on bowed string instruments, by rapidly moving the bow back and forth; plucked strings such as on a harp, where it is called bisbigliando (Italian pronunciation: [bizbiʎˈʎando]) or "whispering"; and tremolo picking, in which a single note is repeated extremely rapidly with a plectrum (or "pick") on traditionally plucked string instruments such as guitar (although a pick is not necessary to execute a tremolo), mandolin, etc.
- between two notes or chords in alternation, an imitation (not to be confused with a trill) of the preceding that is more common on keyboard instruments. Mallet instruments such as the marimba are capable of either method.
- a roll on any percussion instrument, whether tuned or untuned.
A second type of tremolo is a variation in amplitude:
- as produced on organs by tremulants
- using electronic effects in guitar amplifiers and effects pedals which rapidly turn the volume of a signal up and down, creating a "shuddering" effect
- an imitation of the same by strings in which pulsations are taken in the same bow direction
- a vocal technique involving a wide or slow vibrato, not to be confused with the trillo or "Monteverdi trill"
Some electric guitars use a (misnamed) lever called a "tremolo arm" or "whammy bar" that allows a performer to lower or (usually, to some extent) raise the pitch of a note or chord, an effect properly termed vibrato or "pitch bend". This non-standard use of the term "tremolo" refers to pitch rather than amplitude. However, the term "trem" or "tremolo" is still used to refer to a bridge system built for a whammy bar, or the bar itself. True tremolo for an electric guitar, electronic organ, or any electronic signal would normally be produced by a simple amplitude modulation electronic circuit. Electronic tremolo effects were available on many early guitar amplifiers. Tremolo effects pedals are also widely used to achieve this effect. In acoustic instruments, for e.g. guitar, tremolo effect provides the sustenance of sound for a longer span.