Hi all. Studying Bach's Minuet in G, BWV 114 with my son, and noticed in the sheet music I have that there is a trill sign above the C in third measure. The fingering in our sheet music shows "321", so obviously intention is to play with those fingers - but which notes?

Is it CDE? EDC?

Thanks.

Ezra, is that the minuet from the Anna Magdalena Notebook?
If so I think it is a mordant, not a trill. A mordant starts on the main note, goes to the one below, then back to the main note. In this case, C B C. If it is bar 3, The 321 lets you switch end up with your thumb on the C allowing the use of 2 3 4 5 on the subsequent D E F# G.

Mordants have a vertical bar through the squiggle.
Trills had plain squiggles.

Trills usually start on the note above the primary note. A trill on C would be something like D C D C. In some easier pieces it may be suggested to use C D C, which might be more correctly called an inverted mordant.

Some music books have a section in the preface describing the interpretation of the various ornamentation signs. This is especially helpful when you see a long squiggle with a descending loopy at the front and a vertical bar through it near the end.

mordents can be represented with either squiggles (upper mordent) or squiggles with lines through them (lower mordents)

http://en.wikipedia.org/wiki/Ornament_%28music%29#Mordent

trills, on the other hand, can be represented with a squiggle over the note, but the squiggle will extend on horizontally past the given note that it's over - either that, or the note value will be of such a long length that performing a mordent over a trill would seem silly. More typically, trills are represented with the "tr" notation over the note in question.

http://en.wikipedia.org/wiki/Trill_%28music%29