In Rinzai a gradual succession of koans is studied. There are two general branches of curricula used within Rinzai, derived from the principal heirs of Rinzai: the Takuju curriculum, and the Inzan curriculum. However, there are a number of sub-branches of these, and additional variations of curriculum often exist between individual teaching lines which can reflect the recorded experiences of a particular lineage's members. Koan curricula are, in fact, subject to continued accretion and evolution over time, and thus are best considered living traditions of practice rather than set programs of study.