The Trill (also known as The Shake for some reason…) is a musical device which is common to most instruments as well as more accomplished vocalists. It is written as the letters ‘tr’ followed by a wiggly line directly above the note or notes to be trilled and is performed by rapidly alternating the written note with the note immediately above it in the musical scale. If the trill is to oscillate to a note foreign to the key signature, an accidental (sharp, flat or natural sign) would be notated above the note in question. Apart from this, trills are generally diatonic (meaning the interval between the two notes would be in accordance with the key of the piece) and therefore some trills are likely to be semitones and some full tones. If a composer or orchestrator requires a note to be trilled downwards (i.e. with the note below it), this would generally be written a note lower and start with a grace note above (e.g. if a composer wished to trill downwards between the notes C and B then he or she would write a grace note on the note C followed by a written B with the trill symbol directly above it).
For stringed instruments, trills almost always tend to be played slurred. This means that the fingers rapidly alternate notes as the bow continues it’s stroke. It is very rare for a trill to be bowed separately (i.e. the bow changing direction as each note changes). If this was ever required, generally the composer would have to write out the notes in full rather than applying the trill symbol.
Trills can be highly effective in pieces of music written for solo instruments as well as those composed for larger sections of stringed instruments and provide a graceful and often dream-like character to a passage. They are also associated strongly with the 17th and 18th centuries and therefore can often conjure up an antiquated and charming feel to a piece which will evoke associations with an older style.
Although to the listener it would appear that trills require an advanced level of co-ordination to play, in effect they are slightly more straightforward in that generally the upper finger does the trilling whilst the lower finger remains on the lower note. It is also possible to trill between an open string and a first finger.
If a composer or string arranger would like a player to trill between an interval greater than a tone and a half (e.g. a trill symbol could be applied to the note C and by the use of an accidental could enable a trill with a D#, giving a tone and a half), then like the separate bows, this would usually be notated in full. Depending on the speed of the piece, this could be written with demi semi quavers all slurred in the same bow.