In modern musical notation a trill is generally indicated with the letters tr above the staff. This has sometimes been followed by a squiggly line, and sometimes in the past, the sqiggly line on its own was used. The following two notations are equivalent:
The usual way of executing a trill is to rapidly alternate between the note indicated and the note above:
It should be noted that this is an approximation of how a trill might be executed. In many cases, the rate of the trill will not remain constant as indicated here, but will start slower and became more rapid. Whether a trill is played in this way or not is largely a matter of taste.
The number of alterations between notes can vary according to the length of the note in question. At slower tempos a note will last longer, meaning more notes can be played in the trill, but with a fast tempo and short note a trill might be reduced to nothing more than the indicated note, the note above and the indicated note again.
Trills may also be played beginning on the note above the one indicated (the auxiliary note). Additionally, a trill is often ended by playing the note below the one indicated followed by the note itself. This means that an alternative interpretation of the above trill might be
In the Baroque period, a number of signs indicating specific patterns with which a trill should be begun or ended were used. In the Klavierbüchlein für Wilhelm Friedemann Bach, Johann Sebastian Bach lists a number of these signs together with the correct way to interpret them.
Unless one of these specific signs is indicated (and beyond the Baroque period they are very rare), there are no definite rules about whether a trill starts on the main note or the auxiliary note and whether the above alternative ending or some similar alternative beginning is used. These factors, together with the overall rate of the trill and whether that rate is constant or variable, can only be determined by considering the context in which the trill appears, and is usually to a large degree a matter of opinion with no single "right" way of executing the ornament.
The trill is frequently found in classical music for all instruments, although it is more easily executed on some than others. For instance, it is relatively easy to produce a trill on the piano, but on the trombone it is virtually impossible because of the slide mechanism.
The Trill are two symbiotic races of aliens in the fictional Star Trek universe. See Trill (Star Trek).