A closed string is a generalization of a particle. The spatial coordinate of a point on the string is conveniently described by a parameter σ σ which runs from 0 to 2 π π . Time is appropriately described by a parameter σ σ 0 . Associating each point on the string in a D-dimensional spacetime with coordinates x 0 , x and transverse coordinates x i , i = 2 , . . . , D , these coordinates play the role of fields in a 1 + 1 dimensional field theory. Clearly, for such a theory more is required. It is convenient to employ instead of x 0 = σ σ 0 and x , light-cone coordinates x ± ± given by