Polyrhythms run through Brahms’s music like an obsessive-compulsive streak. You don’t have to be musically literate to know the bumpy feel of a cross-rhythm. Two-against-three can be a parent strolling hand in hand with a skipping child. Triplets on top of eighth notes are like a slow canter next to a trot: The two horses might move at the same speed, but you wouldn’t want them pulling a carriage together.