Wind instruments, particularly
woodwinds, can also perform a tremolo.
The most common use of tremolo in wind parts is as a variation on the trill. A trill involves alternating between two adjacent scale tones (thus, the two notes are never more than an augmented second apart). If the composer, arranger, or notator wishes a similar effect that covers a larger interval, he or she will usually notate a tremolo.
Technically, though, a tremolo is not simply a trill with expanded range. While a trill is normally intended to be played as quickly as possible (not always; trills are occassionally slow, or of variable speed, for stylistic reasons), a tremolo is supposed to be played evenly and at a measured tempo.
This type of tremolo is notated as two notes of equal duration but different pitches, with a set of one or more parallel bars angling (up or down) from the first to the second. The entire construction occupies a duration equal to only one of the notes -- so a half note tremolo looks like two half notes but lasts only as long as a single half note.
The number of bars used in the tremolo notation indicates how quickly the tremolo should be played. If there is a single bar in the notation, the tremolo should be played as eighth notes -- the single bar corresponds directly to the single flag on an eighth note. Likewise, two bars indicates sixteenth notes (two flags), and three bars means thirty-second notes (three flags).
Tremolos with three bars are by far the most common. At most common tempos, such a tremolo is so quick that it does sound like a trill with expanded range. Occassionally a notator will use a tremolo to indicate an alternating series of eighth notes or sixteenth notes, but such use is rare.
The other type of tremolo used by wind instruments is rarely seen. Up-angled tremolo bars are placed above or below the note-head (opposite the stem) of a single note. This notation is performed as if it were a series of shorter notes with total duration equal to the tremoloed note. A half note with two tremolo bars, then, would be played exactly the same as eight sixteenth notes at the same pitch.
For comparison, a string tremolo is notated with the tremolo bars directly on the note stem.