A thong (or cord) that has been alum tawed to make the leather pliable while still durable. Used frequently as a binding support both for hard-bound and soft-bound books. Alum-tawed thongs can also be used to closures on books. Alum-tawed skins may be used for soft covers. G. A. Glaister describes thongs as: "strips of leather to which the sections of a book were sewn, after which the ends of the thongs were laced through holes in the boards and secured on the inner side with wooden pegs or wedges. ..."
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Geoffrey Ashall Glaister. Encyclopedia of the Book, 2nd ed. (New Castle, DE: Oak Knoll Press; London: The British Library, 1996).