Reading the works of Bach, there's an unusual pattern I keep seeing. Consider, for example, the opening of the fugue from the infamous BWV 565:
It constantly alternates between A, which doesn't move, and a lower note, which does move. Obviously this is a fugue, so this exact phrase appears again and again — but Bach seems to do this constantly, in just about every organ piece I've looked at. Sometimes the moving note is higher than the stationary note, sometimes it's lower. (In the toccata, one actually crosses the other at one point!)
Is there a name for this perculiar structure?
I know the general name for playing the notes of a chord sequentially is arpeggio. But this clearly isn't a chord, it's a melody. And there's always exactly 2 notes. And they always strictly alternate; never any more complex pattern. And one note stays stationary, and the other moves. And it only ever seems to move in fairly small steps, apart from the odd isolated jump. That all seems quite specific... like it should have a name. Also, as far as I know, only Bach does this. (Having said that, I've only ever tried to play Bach and pop songs, so...)