Ideally, a trill is done with one finger, and preferably a finger that is nimble and independent, like an index or middle finger:
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In many cases that isn’t possible. When two fingers (or more!) are needed, it’s best if they can be fingers of the same hand, moving in the same direction (moving down onto keys/holes simultaneously).
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If the most obvious trill fingering involves more than one finger, try moving them each individually and see if you can produce something that works. If the pitch of the trilled note isn’t quite right, many woodwind players lean toward a sharper upper note rather than a flatter one.
There are good fingering charts available online and in print that offer possible trill fingerings for when common/standard/obvious fingerings don’t work. If you find you need to invent a fingering, a good starting point is to finger the lower note on your instrument, and see what holes there are that you could open with a finger or two to possibly produce the upper note. Try each of them, and some combinations, until hopefully you find one that produces the right pitch. If you have a good understanding of your instrument’s registers, you may also find that you can borrow fingerings for one or both notes from other registers.
Sometimes the tone, pitch, or response of the trill fingering isn’t good when you sustain it as an individual note, but will work acceptably in the context of a trill.
The two notes of the trill should be about equally balanced, so that if you were to record it and slow it down you would hear that the individual notes of the trill are equal in duration and volume. Trills should also fit volume-wise into the context of the musical phrase; use strong and consistent breath support, as though you were playing a single long note.
Trill speed is an artistic decision. Generally trills should be fast enough to give the impression of an effect applied to a single note, rather than a sequence of separate notes. They usually shouldn’t be so fast as to sound jarring or unnatural. The speed of the trill can change for musical effect, and when it does it usually starts slower and accelerates. The best way to learn appropriate trill speeds is by listening to great performances and recordings.
