The university is made up of thirty-nine semi-autonomous constituent colleges, six permanent private halls, and a range of academic departments which are organised into four divisions. All the colleges are self-governing institutions within the university, each controlling its own membership and with its own internal structure and activities. All students are members of a college. It does not have a main campus, and its buildings and facilities are scattered throughout the city centre. Undergraduate teaching at Oxford is organised around weekly small-group tutorials at the colleges and halls – a feature unique to the Oxbridge system. These are supported by classes, lectures, seminars, laboratory work and occasionally further tutorials provided by the central university faculties and departments. Postgraduate teaching is provided predominantly centrally.