The first is a rapid reiteration
- of a single note, particularly used on bowed string instruments and plucked strings such as harp, where it is called bisbigliando (Italian pronunciation: [bizbiʎˈʎando]) or "whispering".
- 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 (somewhat misnamed) device called a "tremolo arm" or "whammy bar" that allows a performer to lower or raise the pitch of a note or chord, which is known as vibrato. This non-standard use of the term "tremolo" refers to pitch rather than amplitude.
Tremolo was invented by the late 16th century composer Claudio Monteverdi and, written as repeated semiquavers (sixteenth notes), used for the stile concitato effects in Il combattimento di Tancredi e Clorinda. The measured tremolo, presumably played with rhythmic regularity, was invented to add dramatic intensity to string accompaniment and contrast with regular tenuto strokes. However, it was not till the time of Gluck that the real tremolo became an accepted method of tone production. Four other types of historical tremolos include the obsolete undulating tremolo, the bowed tremolo, the fingered tremolo (or slurred tremolo), and the bowed-and-fingered tremolo.
The undulating tremolo was produced through the fingers of the right hand alternately exerting and relaxing pressure upon the bow to create a "very uncertain–undulating effect ... But it must be said that, unless violinists have wholly lost the art of this particular stroke, the result is disappointing and futile in the extreme," though it has been suggested that rather than as a legato stroke it was done as a series of jetés.
Instrumental techniques 
|This section does not cite any references or sources. (June 2012)|
The term tremolando especially refers to a rapid repetition on a bowed string instrument, one of the most commonly seen uses of the technique. Tremolo on a violin or similar instrument is sometimes combined with playing sul ponticello (bowing near the bridge of the instrument), which gives a thin and reedy effect, often perceived to be "ghostly."
Another common use of the technique on one note, a reiteration, used in the playing of instruments such as the mandolin. It is common in Russian folk music, used in instruments such as the balalaika, domra and gusli. Once a string is plucked, the note decays very rapidly, and by playing the same note many times very rapidly, the illusion of a sustained note can be created. The technique is also common in the playing of the marimba.
Tremolo is also well known classical guitar technique that involves using the thumb to play single bass notes concurrent with, or directly followed by rapid repetition of a higher note played by two (the middle and index) or three (the ring, middle and index) fingers. Francisco Tárrega notably used this technique in his famous composition Recuerdos de la Alhambra.
Tremolo on two or more notes is common on the piano and other keyboard instruments. The composer Franz Liszt often used the technique in his piano pieces. On the piano, tremolo can create a seemingly louder and larger sound that can be sustained indefinitely. Historically, its use on keyboard instruments can be traced back to a time before the invention of the piano when harpsichords and similar instruments such as the spinet were standard. These instruments could not sustain notes for nearly as long as a modern piano, and so tremolo was used to simulate a longer sustain, as well as being used as an independent effect.
Tremolo can also be achieved through the use of amplitude modulation. This type of effect is often used by electronic instruments and takes the form of a multiplication of the sound by a waveform of lower frequency known as an LFO. The result is similar to the effect of rapid bowing on a violin or the rapid keying of a piano. In accordions and related instruments, tremolo by amplitude modulation is accomplished through intermodulation between two or more reeds slightly out of tune with each other. On organ these ondulating ranks are called celeste or onda maris.
Another device that can produce a tremolo-like effect is a Leslie speaker. This has a (usually large) horizontally-rotating speaker which produces oscillating louder/quieter effect to an individual listener. It does not alter the volume of a note, but produces that effect due to its rotation towards and away from the audience. It is used most often with electric guitar or an electric organ. The musician controls the speed of rotation and the volume, creating a wide range of Leslie effect.
|In musical notation, tremolo is indicated by strokes through the stems of the notes; in the case of semibreves (whole notes), which lack stems, the bars are drawn above or below the note, where the stem would be if there were one. Generally, there are three strokes, except on quavers (eighth notes), which take two, and semiquavers (sixteenth notes), which take one:|
Because this is the same notation as would be used to indicate that regular repeated demisemiquavers (thirty-second notes) should be played, the word tremolo or the abbreviation trem., is sometimes added (particularly in slower music, when there is a real chance of confusion). Alternatively, more strokes can be used.
|If the tremolo is between two or more notes, the bars are drawn between them:
In some music a minim- (Half note) based tremolo is drawn with the strokes connecting the two notes together.
Bowed string instruments 
Violin double stop chords:
Violin bowed tremolo:
Violin fingered tremolo; notice the joining of strokes and stems is different for different time values, and that all notes shorter than eighth notes are written out, such as the last thirty-second notes on the last beat of measure three:
Violin bowed-and-fingered tremolo, notated the same as fingered tremolo but without slurs and with stacc. above the staff:
See also