The most simple structure for a local government system would be the division of a nation into one tier of similarly sized, all-purpose local government units. Such a pattern does not exist in any large liberal democracy, although it is being seriously suggested for Britain. Structures of systems vary in terms of the number of tiers, the comparative sizes of local units and the functions assigned to them. In addition, many nations have not only multi-purpose units of local government, but a number of single-purpose ad hoc local agencies.