Rony＇s extent of separation xi is useful to characterize the quality of separation of a binary feed that occurs in membrane stages, membrane modules and cascades. The stage extent of separation, xi(i) has a maximum value for a specified stage separation factor, alpha(i). The ideal cascade design requires a minimum total interstage flow compressor duty, and membrane area to make a specified overall separation. It is shown that xi(i) has the maximum value at each stage in an ideal membrane cascade composed of ideal perfect-mix stages. This is also true in a "properly designed" no-mix cascade composed of crossflow stages. This suggests that xi is a basic separation index.