The University of Oxford has 38 Colleges and 6 Permanent Private Halls (PPHs) of religious foundation. Colleges and PPHs are autonomous self-governing corporations within the university, and all teaching staff and students studying for a degree at the university must belong to one of the colleges or PPHs. These colleges are not only houses of residence, but have substantial responsibility for student teaching. Generally tutorials (one of the main methods of teaching in Oxford) and classes are the responsibility of colleges, while lectures, examinations, laboratories and the central library are run by the university. Most colleges take both graduates and undergraduates, but several are for graduates only.