There are several versions of superstring theory: type I, type IIA, type IIB, and two flavors of heterotic string theory (SO (32) and E 8 × E 8). The different theories allow different types of strings, and the particles that arise at low energies exhibit different symmetries. For example, the type I theory includes both open strings (which are segments with endpoints) and closed strings (which form closed loops), while types IIA, IIB and heterotic include only closed strings.