Catch structures itself around various combinations of the four instruments (piano, cello, violin and clarinet). There are several games going on: at the start, the clarinet is the outsider, the other three are the unit, then, after a decoy entry, the clarinet takes the initiative. All four then play jovial 'pig-in-the-middle' with each other. The clarinet is then phased out leaving a sullen piano and cello, with interjections based on the clarinet's original tune. This slower passage gradually mutates back into fast music, and this time the game is in earnest: the piano is squeezed out, only to lure the clarinet finally into the snare of its own music.