Any attempt at varying the distances between shelves in every tier will lead to confusion in a strictly classified library. On the other hand, liberal spacing will result in the loss of a shelf in every tier, thereby reducing the total storage space by about one-eighth or one-ninth, according to the number of shelves in a tier. The balance of advantage lies, with movable forms of shelving, and it is strongly recommended that no other kind be specified or ordered. =145.= The following diagrams give the usual dimensions for ordinary standard and wall bookcases, and may be taken as the unit from which a library stack can be built up according to any plan of arrangement. Fig. 31 represents a double-sized standard iron bookcase, 7 feet 6 inches × 3 feet 2 inches × 15 inches, which can be joined end to end to form cases of any length, or used in halves to form cases against walls. Exactly the same dimensions can be used with wooden presses fitted with adjustable brackets or catches. In reference libraries the dimensions may be slightly varied, as the average book which must be stored is rather larger than in lending libraries. But the chief provision for folio and large quarto books should be in special cases arranged round the walls, and it is well to have presses intended for music and quartos fitted with uprights about eighteen inches apart, in order to distribute the weight of the books and facilitate their handling. [Illustration: FIG. 31.--Double Bay Standard Metal Bookcase (Section 145)]. =146.= For standard reference cases the unit of size should be 7 feet 6 inches × 3 feet 2 inches × 18 inches. Special wall-cases should be the same height, but should have an arrangement for large books in the form of a ledged base projecting at least six inches from the front of the upper part of the case, about three feet above the floor (Fig. 32).