Headwear is an important component of cap-and-gown, and the academic costume is not complete without it. The headwear will vary with the level of academic achievement and, to some extent, on the individual academic institution's specifications. For caps, the mortarboard is recommended in the Code, and the material required to match the gown. The exception—velvet—is reserved for the doctor's degree only, seen in the form of a multiple-sided (4, 6, or 8) tam, but the four-sided mortarboard-shaped tam in velvet is what the Code seems to recommend here. The only colour called for is black, in all cases. The tassel worn on the mortarboard or a tam seems to provide, by tradition, the greatest opportunity for latitude in American academic dress. It has been black, or represented the university's colours, or the colours of the specific college, or the discipline. The tassel has also been used to indicate membership in national honour societies or other awards. There is at some colleges and universities a practice of moving the tassel from one side to the other on graduating, but this is a modern innovation that would be impractical out of doors due to the vagaries of the wind. For doctoral and masters students, the tassel commonly begins and remains on the left.