A braid is an intertwining of some number of strings attached to top and bottom "bars" such that each string never "turns back up." In other words, the path of each string in a braid could be traced out by a falling object if acted upon only by gravity and horizontal forces. A given braid may be assigned a symbol known as a braid word that uniquely identifies it (although equivalent braids may have more than one possible representations). For example, is a braid word for the braid illustrated above.

If is a knot and

where is the Alexander polynomial of , then cannot be represented as a closed 3-braid. Also, if

then cannot be represented as a closed 4-braid (Jones 1985).