A kind of ornament. It is notated with the lowercase letters ‘tr’ on top of a note. So when we see a crotchet, for instance, with ‘tr’ on top of it or right below it, we know that we have to perform fast alterations between the note that we see and the one next to it, higher in frequency, while remaining on the scale we’re in. Now, this can be a bit tricky as there are quite a few rules surrounding trills, but things will start making sense soon enough. Let’s say that we are in G major, as it’s a scale that has only one sharp in its key signature, F#. In case we have a trill above the note E, we need to alternate between notes E and F# (as F# is in G major, not F natural, ie. F♮). In case we have an accidental above or below the letters ‘tr’, we know that the note on top probably isn’t in the scale we’re currently in. For instance, in G major, if we see on top of note E the letters ‘tr’ and above that the sign of a natural, then we would alternate between notes E and F, instead of E and F sharp. Usually, we start a trill with the lower note and go 2-3-2-3 instead of 3-2-3-2, but that depends on the era and genre of the composition… Also, trills can end in more interesting ways, by adding a note below the ones of the trill. For instance, again in G major, we would alternate between the notes E and F# and end the trill with a D followed by another E, doing something like: 2-3-2-3-2-3-2-1-2. Kinda starts to make sense now, doesn’t it?