Peter Lombard's primary contribution to the development of medieval theology was the basic textbook, the Sentences, noted for its comprehensiveness, clear structure, and quotations from previous theologians. On the basis of this text, Peter Lombard was known throughout the rest of the middle ages as the "Master of the Sentences." Many well-known theologians after him held the post Sententiarius in various universities, an appointment which obliged them to comment on the Sentences. Among the first theologians to settle on the number seven for the sacraments, Peter Lombard anticipates later developments regarding the efficacy of the sacraments.