The American tradition of moving the tassel on graduation caps from the right side to the left side during the commencement ceremony signifies the recognition of earning a degree. It is sometimes called "turning of the tassel."
Although the tassels are sometimes turned by all graduates at once, it is often done when the student is physically handed her diploma.
While graduation tassels come in many colors, the American Council on Education states that each tassel should be black or the color of the subject the degree was earned in. The only exception is for those who have doctoral degrees; their tassel is gold.