A trill is an ornament that alternates rapidly between two neighboring notes.
How to use it
Add a short one on a long note or at the end of a phrase, and adjust the speed so it fits the tempo.
Difference from related terms
A trill is the repetition of two notes, while tremolo refers to the repetition of a single note or a wavering of volume.
It sounds natural when its length and level stay under the main melody.
