Horizontal bar in trill fingering

Question

I'm looking at the Henle edition of Bach's WTC, specifically at d-moll Fugue (BWV 851), and in the second measure I see a trill:

(This is András Schiff's fingering). What is that horizontal bar over numbers 3 and 2 supposed to mean? I guess, Schiff means here something like what is described in this answer (see Option 1), namely, play A with 3rd finger, then G with 2nd finger, iterate this for some time (and this is what the bar actually means), and then go to F with 1st finger, and again G with 2nd. But I am not sure (partly because this notation does not give specific notes, but the fingers only). So,

1. Am I right in my conclusion that the bar means "repeat these two notes for some time"?
2. A meta-question: how was I supposed to know this? Is it in some (or every) piano textbook and everyone just learns it in fourth grade or something? Is there a canonical source of widely accepted notation regarding fingering in the modern piano literature, or in Henle editions in particular? By the way, my edition includes "Table of ornaments", which shows how to play various "idems" and "Doppelt-Cadences", but it does not mention a common trill, and my guess looks more like "Trillo und Mordant" in this table.

Answer 1

Question One: yes, this is one of most-liked "classical" or "canonical" ways to play this kind of ornament. IIRC then at Bach's time there was no direct trill (as we have now), but he had his own set of ornaments. There are extensive descriptions of how to the play these ornaments by the master himself -- it must have been important to him. I have these descriptions in one of his books, but I'll have to look it up which one it is. Maybe in Inventionen or in Preludes.

As a side note, my guess is that more or less every student struggles with the correct interpretation, which can never be achieved (student meaning everyone except the master himself).

Question two: afaik your teacher "has to know". When I practiced for University, I had to learn all the canonical ways to play the ornaments in Bach. There seems to be a secret agreement within the high temples of musical Universities how to do this and enforce this by accepting no one who plays it differently. I really don't know how this gets decided, what the (supposedly) correct way of playing is. If you listen carefully to different recordings you'll notice that there is a wide range of possible interpretations how to ornament.

And as a last note, there is extensive literature on the use of ornaments in Bach's music, including instructions how to play it.

Answer 2

I have also been looking for an explanation of what the horizontal line over the fingering numbers for an ornament means (as it appears in G. Henle publications).

My best guess is that such a mark is used somewhat like a vinculum to indicate a repeating group.

So, for example, in the fingering by Michael Schneidt for G. Henle's urtext of Bach's two-part inventions I have seen some trill signs given four notes/numbers (such as 3231) elsewhere I have seen just two (such as 32 with the horizontal line above them). In both cases, the ornament appears over an eighth note- leading me to believe there is an intention to perform four notes either way.

Maybe a line over a group of numbers (so far I've only seen it over a group of two)in the fingering of an ornament means repeat that group. In other words, a line over 32 means 3232 (if the ornament is written over an eighth note- 323232 if written over a quarter note)?

I don't know for certain. This is just my best guess.

Have you found an answer elsewhere to your question?

Also, I noticed that your original example has the ornament written above a quarter note which would (according to Bach's table of ornaments- which G.Henle refers to) allow enough time/space for 6 notes to execute a trill.

Finally, perhaps the answer to your original question above is: 323212 (?)