Alterations over trill?

[thierry13]

What does the alterations over trill mean? There sometimes are flats/double-flats/naturals over some trill notations. What do they mean? What is the alteration of the just over your current note to know if you will trill with using a half-tone over or a full tone over?

[cadenz]

i think it works like this:
you generally trill with the note that is under the trill written on the stave, and with the note  that would be on the line above the written note, if that line is # in the key signature then that would be sharp too. if above the trill is a flat or a natural or a sharp, this only effects the note above the written note.

does this help?

[whynot]

The "default" trill note is the next note up in whatever scale your piece is in.  But sometimes that note needs to be adjusted slightly, either to better fit a harmony or to lead differently to what comes next.  This adjustment is shown by accidentals with the trill sign.  So first determine the next note in the scale, and then make the adjustment (natural it, flat it).  Turns work the same way:   determine what notes are just above and below the written note according to the scale you're in, then apply any accidentals given with the turn symbol.  If the accidental is above the turn symbol, that change is for the upper note.  If it's just below it, that's for the lowest note in the turn.