It was earlier shown that an SO(9,1) θ α spinor variable can be constructed from RNS matter and ghost fields. θ α has a bosonic world-sheet super-partner λ α which plays the role of a twistor variable, satisfying λΓ μ λ = x μ + iθΓ μ θ. For Type IIA superstrings, the left-moving [θ α L , λ α L ] and right-moving [θ R α , λ R α ] can be combined into 32-component SO(10,1) spinors [θ A , λ A ]. This suggests that λ A Γ 1 1 A B λ B = 2λ α L λ R α can be interpreted as momentum in the eleventh direction. Evidence for this interpretation comes from the zero-momentum vertex operators of the Type IIA superstring and from consideration of D 0 -branes. As in the work of Bars, one finds an SO(10,2) structure for the Type IIA superstring and an SO(9, 1) SO(2, 1) structure for the Type IIB superstring.