The Square Graduation Cap

... also known as a mortarboard hat, is a four-cornered, tasselled black cap that is part of the scholarly tradition and is worn on graduation day, marking the academic accomplishment.

It has originated from the medieval times, when certain caps were used by European scholars, back in the 11th century. The shape of the cap kept changing, and in the 16th century, this square cap was adopted by the clergy.