Tremolo

Tremolo is a musical term with several meanings:

A regular and repetitive variation in amplitude for the duration of a single note. This is the most common meaning, but is often considered a misnomer.
A regular and rapid repetition of a single note, which is scored as a single note, and particularly used on plectrum instruments such as the balalaika and the mandolin family. On these instruments it is more often called a trill, but on electronic organ stops imitating these instruments it is generally called tremolo.
A regular and rapid alternation between two notes, which is scored as a trill.
A roll on any tuned or untuned percussion instrument.
A variation in pitch, slow or rapid, during the duration of a note. These techniques are more normally called portamento and vibrato.

Some discussion of the last sense given above can be found at tremolo arm and vibrato, and a detailed discussion of the terminology used by electric guitarists and its history at vibrato unit. The rest of this article is concerned with the more generally accepted meanings.

Tremolo is the rapid repetition of one note in music or a rapid alternation between two or more notes. It is sometimes called tremolando, especially when referring to a rapid repition 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 (over 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 is in the playing of the mandolin. Once a mandolin 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 on two or more notes is most frequently seen on the piano or other keyboard instruments. The composer Franz Liszt often calls for the technique to be used in his piano pieces. When used on the piano, tremolo can create a seemingly louder and larger sound, which 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 wave form of lower frequency. 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.

Notation

In music notation, tremolo is indicated by strokes through the stems of the notes (in the case of semibreves or 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: