Archive catalogues are structured in terms of "levels." It may be helpful to imagine the filing system of a computer, with folders and sub-folders corresponding to levels and sub-levels. The main levels are:
The whole of the collection, organically created and/or accumulated and used by a particular person, family, or corporate body in the course of that creator's activities and functions. The collection level provides an overview and a summary of the contents of each archive collection, e.g. the Cambridge to Ely Turnpike Trust Collection.
A subdivision of the Collection level, e.g. the turnpike secretary's papers.
A series of records within the collection, e.g. the minute books created by the secretary.
A group of related records within the series, e.g. an individual minute book or a photograph album.
The smallest intellectually indivisible archival unit within the series, e.g. a letter within a bundle.