Every spring, U.S. high schools, colleges and universities celebrate student graduations. The most common headgear at these functions is the square, flat-topped mortarboard—but when did graduates start wearing it?

Here’s a look at the history of these ceremonial caps.

Founded around 1088, the University of Bologna in Italy is recognized as Europe’s first university and remains the world’s oldest continuously operating institution of its kind. By the mid-12th century the University of Paris and Oxford University had also been established, continuing the spread of higher education in Europe.

Early students at these universities may have worn a calotte or skullcap similar to what some church figures wore. Later, students adopted a brimless hat called the pileus. Named after a similar cap from ancient Rome, the pileus was the progenitor of the modern graduation cap.

In the 16th century, two versions of the pileus became popular with university graduates: the square-shaped pileus quadratus and the circular pileus rotundus. Students might wear one or the other depending on what subject they studied. In 1675, the English engraver David Loggan published an illustration of Oxford academic regalia that included both flat, square-shaped caps and rounded ones.

This academic dress traveled across the Atlantic in the 17th century as English settlers established colleges in the American colonies. In the United States, the flat-top caps eventually became known as “mortarboards,” a term that originally referred to an actual square board used to hold mortar. In the United Kingdom, you might hear people call them “trenchers.”

Loggan’s 1675 illustration of Oxford caps didn’t feature any tassels. But at some point, graduates began attaching these decorations to the buttons on top of their graduation caps. By 1895, tassels were common enough for U.S. colleges and universities to write in the Intercollegiate Code of Academic Costume that all graduation caps should have them.

Whatever the tassel’s origins, it now plays a symbolic role in U.S. graduation ceremonies for high schools, colleges and universities. During many of these ceremonies, students move the tassel from right to left across their cap to mark their achievement.

The 1895 Intercollegiate Code of Academic Costume specified that graduation caps should be “mortar-board,” indicating that this was probably already the most common style in the United States at that point. As high schools began to offer diplomas and hold graduation ceremonies, mortarboard caps became a part of these celebrations too.

However, if you’ve been to a university graduation lately, you might have noticed that students receiving graduate degrees have different headgear. Just as 16th-century students wore different caps depending on their field of study, students earning a graduate degree might wear something called a “tam.” These tams can have a square, hexagon or octagon shape, depending on the type of degree they represent.