Abstract
We generalize the gauge-invariant theory of the free bosonic open string to treat closed strings and superstrings. All of these theories can be written as theories of string differential forms defined on suitable spaces. All of the bosonic theories have exactly the same structure; the Ramond theory takes an analogous first-order form. We show, explicitly, using simple and general manipulations, how to gauge-fix each action to the light-cone gauge and to the Feynman-Siegel gauge.