 CONT: It becomes fairly obvious during the "Fated to be Mated" duet  between Fred Astaire and Cyd Charisse that Charisse is wearing a  skirt one moment and culottes (or flared shorts) the next. The  bottom half of her costume changes on each cut of the dance when  they are doing deep knee bends, and this is where the culottes show.  For the upright spins and lifts, the skirt shows. The dance was  obviously performed twice and edited into one sequence.