I’m trying to understand the use of brackets that show which hand is used to play a part in piano notation. I was reviewing Bach’s BWV 888 Fugue on Henle’s website and noticed that the brackets were only placed before a note, not after them. Does that mean brackets function similar to accidentals, that is they are in effect until the end of the measure? Are there rules or is it specific to the composer?
No. They simply indicate when it is convenient (sometimes necessary) to take a certain line of counterpoint with one hand or the other. The reason there is only one is that each note in that voice will continue in the indicated hand until it’s obvious that you should switch.
I’d be very judicious about using these if I were you, especially if you don’t understand them well. I consider them much like fingerings (indeed, these brackets interact and affect fingering). It can be very personal and hand sizes are different. Often they aren’t necessary, or there’s more than one way to do it, so it’s fine to allow performers to make their own marks.