A single letter engraved or cast into a piece of metal tool fitted into a wooden handle.
Roberts, Matt, Don Etherington, and Margaret R. Brown. 1982. Bookbinding and the conservation of books: a dictionary of descriptive terminology. Washington: Library of Congress.
Bernard C. Middleton (1998), The Restoration of Leather Bindings, 3rd ed. rev. & expanded, New Castle (DE); London, Oak Knoll Press ; British Library.