What is the origin of mortarboards ?

They were Medieval clerical caps, all (?) the original European universities being religious. They evolved into a square top, which resembles a bricklayer's mortarboard.

From The Word Detective 7/23/02:

"The term 'mortarboard' for these hats first appeared in the mid-19th century, and comes from the resemblance of the hat to the small square board or metal plate on which a bricklayer carries mortar. Earlier, in the 17th century, they were called 'trencher-caps' or simply 'trenchers,' from their resemblance to a 'trencher,' an antiquated sort of dining-plate. And yet earlier they were known as 'catercaps,' from the French 'quatre' (four), referring to the hat's four corners.

"The 'mortarboard' hat itself began as the "biretum," a rounded cap topped by a small knob worn by the clergy in the 13th century. Adopted by lay authorities and academics, it gradually took on a more squared shape, finally in the 17th century reaching its modern "board on a skullcap" form, with the knob having been replaced, in a final bizarre touch, by a tassel."


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