In string theory, the heterotic string is a peculiar hybrid of a 26-dimensional bosonic string and a 10-dimensional superstring. In order for the theory to work correctly, the 16 mismatched dimensions must be compactified on an even, unimodular lattice of rank 16. There are two such lattices: Γ>⊕Γ and Γ (constructed in a fashion analogous to that of Γ). These lead to two version of the heterotic string known as the E×E heterotic string and the SO(32) heterotic string.
