The academic cap or square, commonly known as the mortarboard, has come to be symbolic of academia. In some universities it can be worn by graduates and undergraduates alike. It is a flat square hat with a tassel suspended from a button in the top centre of the board. The mortarboard may also be referred to as a trencher cap (or simply trencher). The tassel is composed of a cluster of silk threads which are wrapped together with a cord which is attached to the button affixed to the centre of the headpiece. The loose strands are allowed to fall freely over the board edge, typically falling over the left front side of the cap. Often the strands are plaited together to form a cord with the end threads left untied. In many universities, holders of doctorates wear a soft-crowned, round-brimmed headpiece known as a Tudor bonnet or tam, rather than a trencher. Other types of hats used, especially in some universities in the UK, are the John Knox cap (mostly at Scottish universities), the Bishop Andrewes cap (a reinvention of the ancient form of the mortarboard, worn by Cambridge Doctors of Divinity DD's) and the pileus (at Sussex). In some universities, such as Oxford, women may wear an Oxford ladies' cap.