I am working on the Ouverture, JS Bach BWV1067, B minor. The first part is 4/4, but at measure 20 it changes to a "2" with a vertical slash. It still contains the equivalent to four 1/4 notes, but it is played considerably faster, probably two times faster than the previous section.
oh weird, just pulled up the score. I've never seen that type of notation, but I'm pretty sure it's just a metric modulation to 'cut-time', which would be twice as fast. This would be similar to the difference between the notation C for common time, and C with a vertical line for cut-time.
Cut-time... OK, I can investigate further now I know the name,thank you. And yes, it could be twice as fast as the first partaccording to the recorded versions I have. This notation appears on Bärenreiter score BA 6812, "Ouverture In B minor for Traverse Flute and Harpsichord Obbligato (Piano)" The arrangement is by Gerhard Kirchner.
Bach did not seem to always put tempo indications on his music. Examples include 3rd Brandenburg concerto and A minor violin concerto. I am guessing that he expected the musicians to know how fast a particular piece was meant to go because it would be the done thing at the time. In this cse, you are referring to a French overture. A French overture has a slow section and a fast section and musicians of the time would know that and just get on and play it. I note from my score that the other suites also start with overtures that have no tempo marking. Also, if the musicians of the time didn't work it out for themselves, Bach would have been there to tell them. Hope that helps.
You are right. The score is ana arrangement without tempo indications.
As an amateur, I have a limited feeling of how it should be played and in practise I normally play slower than I should, at least while I am learning it. I have listened to several versions of this work and the only idea I have is what others have been doing. I am preparing a synthesized piano accompaniment for the first movement and had to tell the software the tempo I wanted. This notation of the slashed "2" was new to me, the software did not understand it and I had to correct the tempo. But now it is clear, thank you.