These are stretchy lycra Renee dresses. Lycra is very similar to elastic, which is used to make tight-fitting garments. Lycra and spandex are technically the same material. The generic version of lycra fiber is called spandex. Lyrca is a slightly better name-brand version of the generic spandex fiber. Fun fact, the name spandex is only used in the good ol' U.S. of A. The rest of the world uses the word elastane. Basically, there are no differences between cotton/spandex blends or cotton/lycra blends.