Devised by BASI Pilates founder Rael Isacowitz, the BASI Block System is a specialized ‘filing’ system for the hundreds of exercises in the Pilates repertoire. It categorizes them in a logical sequence that ensures the progressive structuring of classes within a flexible framework. The Block System encourages progressive thinking within a structure. It enables the instructor to construct a class that is balanced and addresses all planes of movement and multiple muscle groups. The criteria which come into play when choosing an exercise include things like the experience, the level and goals of the client, the needs of the class such at the time, and the structure and flow.