The concept of a tier is central to the structure of local government. However there has been no clear definition of the concept, and the term has been applied in vague and inconsistent ways. Furthermore, no procedure exists for identifying the number of tiers in a local government system. In order to resolve these problems a pure model of a hierarchical local government tier structure is outlined, and the concepts of parallel, partial and comprehensive tiers are introduced and defined. This framework is then applied to the British debate about unitary authorities. It is shown that these units are not single-tier authorities but form part of a vertically fragmented structure. In consequence many of the arguments used in favour of the current structural changes to British local government are not valid because they assume that the reforms will create single-tier units.