Wooden planks obtained from the trunk of a tree inline with the medullary rays radiating from the centre of the trunk, with the growth rings at right angles to the sides of the planks. Such planks were procured either by splitting or sewing the wood along the medullary rays.
Roberts, Matt, Don Etherington, and Margaret R. Brown. 1982. Bookbinding and the conservation of books: a dictionary of descriptive terminology. Washington: Library of Congress.
Christopher Clarkson (1996), “Further Studies in Anglos-Saxon and Norman Bookbinding: Board Attachment Methods Re-Examined”, in Roger Powell, the Compleat Binder: Liber Amicorum, edited by John L Sharpe, 14, Bibliologia, Turnhout, Brepols, pp. 154–214.