Students of classical music may look in vain for scholarly literature on the trill. But the search for its raison d’être is not a trivial pursuit.
There must be answers to the question why passages of quick alteration between two adjacent notes are such an important element in classical compositions, often a few bars ahead of the final chord. In concertos, trills are mandatory at the end of the cadenza, before the orchestra comes in for the conclusion. If prolonged suspense is required, why, of all things, should imitations of bird-songs be used to provide it?
Trills are, of course, also used in the body of compositions – far from their conclusion. They are occasionally even used in a main theme where they do not necessarily suggest hesitation or suspense. If they were substituted by a single note it would be clear that there was something missing.
Please don’t think that trills are primarily used to give the performer an opportunity for showing off.
Nor that their real purpose is to create difficulties for anyone trying to sing or whistle the tune.
Trills are, in fact, a significant part of the musical vocabulary. But until this posting was written, did they get any recognition?