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Tremolo is an effect in music, that generally refers to rapid reiteration or change of notes. It also can mean a change in how loud a pitch is. When tremolo refers to pitch, the first approach is to play one note or chord over and over again. On members of the violin family, for example, players achieve the tremolo by moving the bow rapidly back and forth across the string; they play the note or chord without changing fingerings. On an instrument such as a guitar or mandolin, the player usually uses a pick to repluck the string. These types of tremolos also include rolls on percussion instruments.
Tremolos not of the single-note or chord variety are called fingered tremolos and are very similar to trills. A trill, however, is a rapid alteration between pitches a half step or whole step apart. A fingered tremolo involves intervals of at least a minor third or more. These types of tremolos are especially common on keyboard instruments, although other instruments can play them, as well. Brass players often have the most difficulty performing these tremolos well, simply because the pitch on a brass instrument is defined by the formation of the lips and mouth cavity — that is, the player's embouchure — in addition to the valves used, and embouchure is hard to change back and forth quickly.
In terms of music notation, composers have to have a way of telling musicians when to use tremolo. The simplest way composers do this is to write "tremolo" or the abbreviation "trem." above the staff. In modern notation, however, composers further clarify the desire for the effect through slashes. On a single-note or chord tremolo, the composer draws one slash through the note stem or stem area for 16th notes, two slashes for eighth notes, and three slashes for greater values.
For fingered tremolos, notation is a little more complex. The composer has to write both pitches involved, which can make it look like there are more beats in the measure than there actually are. For instance, if a composer needed the effect to last two beats, he would have to write a half note for the first pitch and a half note for the second pitch. Then he connects both pitches with bars that function the same way the strokes do for single-note or chord tremolos. The bars are between the notes but don't actually touch them, which makes them distinct from the musical beams that define the duration of the note.
Less commonly, tremolo means quickly making a pitch louder or softer instead of playing the same pitches or pitches again. Musicians frequently call on electronic systems to achieve this effect. Amplifiers also are useful in creating these amplitude tremolos.
A common error is to confuse tremolo with vibrato. The first term involves one or more constant frequencies. The second involves very small frequency changes — that is, pitch change. Part of the confusion stems from guitars that feature special effects arms. These are called "vibrato arms" despite the fact they change volume, or amplitude, instead of frequency.