Both the Bonnange and Staderini methods share in common an advantage of some importance, viz., the clamped lower portion of the card forms a counterfoil to show what has been taken, should a card by accident or design be removed or torn off. The accession number or brief title of the book can be written on the clamped portion of the card, and so will safeguard against loss and imperfections. This is an advantage not possessed by any of the ordinary card methods, because when cards are torn from the rods they leave no trace, and become lost for ever, leaving it very problematical whether a catalogue is perfect or not. =281.= A card catalogue on a somewhat similar principle to the French and Italian forms just described is known as the Duplex Card Catalogue, and was invented in England to enable both sides of the cards to be used, thereby considerably enlarging the capacity of the catalogue, while materially reducing its bulk. It is fitted with falling ends which act as angle-blocks; a travelling angle-block can be adjusted and locked at any point; a locking-rod for threading the cards upon in order to secure them; and xylonite label-holders. The cards are larger than ordinary catalogue cards, and instead of being hinged are simply creased at a short distance above the rod holes. This gives a slight bulge and enables the cards to have the necessary play. The trays are held lengthways in a position parallel to the body, instead of at right angles as in the case of ordinary trays, and the cards or leaves are simply turned over like those of a book. An extended, illustrated study of the whole of card-cataloguing methodology is Sayers and Stewart’s _The Card Catalogue_. [Illustration: FIG. 101.--Duplex Card Catalogue (Section 281).] =282. Sheaf Catalogues.=--The sheaf catalogue is not so widely used as the card system in Britain, but, as we have shown, it has exactly the same advantages as regards the power of expansion and intercalation. It aims at combining the advantages of both book and card catalogues, by dividing the catalogue into handy sections so that the maximum number of readers can consult it at one time; providing means for continuous expansion in alphabetical order; safeguarding the contents of sections; reducing the amount of storage space occupied; and enabling users to handle and turn over the catalogue like the leaves of an ordinary book. The introduction of ordinary paper slips, which can be used in any typewriter, which can be easily stored in various forms of binders in book form, and which can be added to in manuscript without undoing the holder, is a real economy in library administration which has not received the attention it deserves. While 1000 entries in a card catalogue will occupy from 750 to 840 cubic inches of space, the sheaf-holders most in use will not take up more than fifty-six cubic inches of space for the same number of entries. The writing surfaces are also much larger.