Questions about mordents and trills (piano)

The difference between an upper and lower mordent is the little like. Notation for an extended trill is quite different.

Now, how to interpret the mordent signs in different styles and eras of music is a whole study in itself!

To your question about F Eb F versus F E F, the general rule is to stay within the key signature unless otherwise indicated.

The term you have to research is ornament (trill, mordent, praller): the trill = upper changing note, mordent = lower changing note.

Here‘s the result of my google research:

In the free downloadable editions of Bach‘s keyboard music the ornaments and their performance in the time of the composer are described in the foreword.

The use and performance of the ornaments (up or down, semi tone or whole, short or longer trill) has been changed in different epoques, eras, generations, regions, styles, countries.

Your first point: questioning for a rule ... this can depend of the movement in the melody, (preceeding interval, position in the piece, mood and fingerset of the performer... I sometimes decide in the last second about ornamentations or don‘t decide at all and just let the fingers play what they want.

I think you mean there's only a tiny difference between an upper and a lower mordent, don't you? That's true.

It is hard to see, but it's a lower mordent, so you might play F Eb F.

In baroque music mordents of both kinds might begin on the unwritten note. So in your example you might play Eb F Eb F. An upper mordent sign can also indicate a short trill, by the way.

I keep saying 'might' because it's not known precisely what each composer intended. If you want to pursue the subject in depth The Interpretation of Music by Thurston Dart might be a good place to start. It may tell you if there's a 'general rule, from the context, when each is used.'

If the piece is by Bach an answer (of mine) to a similar question contains links to the guide to ornaments he wrote for his son Wilhelm Friedemann.