Also known as a mortarboard, Oxford cap, corner-cap, square or trencher, the academic cap has its origins in the Roman Catholic church. It is thought to have developed out of the Italian biretta, which is still worn today by some members of the Catholic clergy. The early academic caps were typically red (a symbol of blood and life), and over time became associated with humanists, students, and artists.
By the 15th century, caps had been incorporated into the official dress codes of many universities. Initially, they were reserved only for those who held master's degrees (the highest qualification in medieval times); however, they gradually began to trickle down into undergraduate wardrobes. In some universities, undergraduate and doctoral headwear still differs, and recipients of doctoral degrees may wear Tudor bonnets (round, soft-brimmed caps) instead of mortarboards.
While centuries ago, caps may have been sported on a daily basis along with other academic garments, today they are worn almost exclusively at graduation ceremonies in most universities. Some institutions, particularly older ones such as Oxford and Cambridge, require students to wear academic dress on a number of other formal occasions as well, such as matriculation (pictured below).
The tradition of throwing your graduation cap into the air is a fairly recent one - its first recorded instance was in 1912 at the US Naval Academy in Annapolis, Maryland. According to one story, the graduates were presented with their officers' hats, prompting them to throw their old midshipmen's caps into the air.
Although they aren't enforced, the American Council on Education (ACE) has developed a list of guidelines for hat etiquette and tassel color. They find that there is no general rule for tassel position on the mortarboard, but "numerous institutions have adopted the practice, during commencement exercises, of requiring candidates for degrees to wear the tassels on the right front side before degrees are conferred and to shift them to the left at the moment when degrees are awarded to them." The ACE asserts that the tassel should be either black, or in the color associated with the graduating student's discipline (e.g. arts is associated with white, law with purple, nursing with apricot. For a full list, click here).
Whether it's at a high school or college graduation ceremony, most people will run into an academic cap at least once in their lifetime. Now you know a little more about them, and where they come from.