"This is the Library's 'three-figure' classification scheme which is used to determine the subject stem of the classmarks of the great majority of monographs on the open shelves. (It is also used in a simplified form in combination with various prefixes for special types of material: R, the reference collection in the Reading Room; S, 'select' books on closed access; P, L, T, Q, primary, secondary, select and dead periodicals, respectively; Z, mutilated books. Some other open access collections, such as music and reference collections in specialised reading rooms, have their own schemes.) This edition represents current practice and interpretation; inspection of the shelves will reveal classes superseded by more detailed subdivision or abandoned entirely, while policy on some of the less specific categories or on new areas of study will be found to have evolved over time. In recent years it has rarely been possible to reclassify in accordance with such changes and developments."