The square academic cap, graduate cap, cap, mortarboard (because of its similarity in appearance to the hawk used by bricklayers to hold mortar) or Oxford cap, is an item of academic dress consisting of a horizontal square board fixed upon a skull-cap, with a tassel attached to the centre. In the UK and the US, it is commonly referred to informally in conjunction with an academic gown, as a “cap and gown.” It is also sometimes termed a square, trencher, or corner-cap. The adjective academical is also used. (source)