The stiff covers at the front and back of a book. Wood was the material generally used until the early sixteenth century, preferably oak or another hardwood to minimize worming. These covers could be very thick and often had bevelled edges. Pasteboard became popular in the sixteenth century; from the late seventeenth century on, it was supplemented by rope-fibre millboards. Strawboard first came into use in the eighteenth century. The boards were attached to the quires by the cords, which were threaded through the boards and secured (often by means of pegging). The boards and spine were then usually covered with damp leather (although parchment, fabric, or paper might also be used), which was folded over the edges of the boards (forming what are known as turn-ins) and glued down. pastedowns could then be applied to conceal this mechanism.