English law, as limited or modified, applies as one of the residual systems of law in Northern Nigeria, the other being customary law. The assumption underlying both the reception of English law and the retention of customary law was that both would be eroded and a new system would be created by local legislation and local decisions. While some erosion of both systems has taken place, such a process is necessarily slow and there remains a large area of law where the relationship of the received and retained law to local legislation is difficult to elaborate with any degree of precision. This is particularly true of the relationship of English law to the provisions of the Land Tenure Law and its predecessor, the Land and Native Rights Ordinance. The primary difficulty encountered in this context is the analogy which can be drawn between a lease in English form and the statutory right of occupancy, since the latter interest is granted by the appropriate authority normally, although not necessarily, for a definite term of years and may be governed by covenants which relate to rent, use and occupation, assignment and subletting, recovery of possession and so on.