The British Museum public catalogue consists of large guard books, in which printed or manuscript slips of book entries are mounted on the tough cartridge paper leaves, so as to leave space for additions. When a page becomes congested, the slips can be lifted by means of a paper-knife, as they are secured only at the ends, another leaf can be inserted on the adjoining guard, and the old and additional slips can be redistributed over the whole of the newly created space. This catalogue represents but one alphabet, or copy of the catalogue, in some hundreds of volumes, and each volume only holds a small portion of the alphabet, as from Bal to Bec. One copy of the catalogue thus serves many readers at one time. By distributing the entries over a number of volumes, congestion is less likely to occur than in catalogues complete in themselves in one or two volumes. [Illustration: FIG. 92.--Catalogue Shelves, British Museum (Section 269).] A variation of this system of guard book is to be seen in some public libraries where the whole of the catalogue is mounted in one volume. A number of copies of this style of page catalogue must be provided to meet public needs, and it is, on the whole, a less serviceable and much more expensive form than the catalogue on similar lines spread over a number of volumes. A good example of this kind of page catalogue is to be seen in the Mitchell Library, Glasgow, where it exists in the form of huge guard books displayed on special stands. =270.= To overcome the difficulty of inserting additional leaves at pleasure in page catalogues, various kinds of adjustable albums, with movable leaves, have been introduced. At the Bibliothèque Nationale, in Paris, and elsewhere a catalogue is used consisting of thick, hinged leaves, punched at the back and laced into the boards, or secured by means of a screw fastening. [Illustration: FIG. 93.--Adjustable Screw Binder (Section 271).] =271.= Another variety of this French binder designed to secure adjustability of leaves is that shown below in the illustration (Fig. 93), wherein the leaves are clamped by the pressure of two wooden slats, which are drawn together by means of two or more endless screws turned by a key. For this kind of binder it is necessary to notch the leaves to correspond with the screws. The principle of the sheaf binders (Section 282) can also be applied to page catalogues, and very successful page books have been made up from the form illustrated in Sections 286-87. The most recent methods of loose-leaf ledgers have also great possibilities for catalogues, and are probably to be preferred to any book manuscript type other than the sheaf.