Ornaments · Music theory
Appoggiatura, mordent, turn, trill and arpeggio: how they are written and how they are played.
What they are
Ornaments decorate a main note. They are written with small notes or signs and do not count when adding up the measure: their time comes from the note they accompany. Exactly what is played depends on the period, so what follows is the most usual reading in the classical repertoire.
If a sign has an accidental above or below it, that accidental affects the auxiliary note, the upper or the lower one.
A practice that changes with the period
There is no single way of playing each ornament. Carl Philipp Emanuel Bach, in the 18th century, held that works with few markings should be ornamented according to custom, and at the end of that century there was still no uniform criterion: different ornaments could be played alike.
So, when in doubt, it matters what period the work is from and what the edition says. A teacher or a good critical edition usually shows how to play them. What is set out here is what a musicianship exam asks for.