Apart from that there exist seven more heterotic string theories which are not supersymmetric and hence are only of secondary importance in most applications.
[1] Heterotic string theory was first developed in 1985 by David Gross, Jeffrey Harvey, Emil Martinec, and Ryan Rohm[2] (the so-called "Princeton string quartet"[3]), in one of the key papers that fueled the first superstring revolution.
The mismatched 16 dimensions must be compactified on an even, self-dual lattice (a discrete subgroup of a linear space).
There are two possible even self-dual lattices in 16 dimensions, and it leads to two types of the heterotic string.
Because the various superstring theories were shown to be related by dualities, it was proposed that each type of string was a different limit of a single underlying theory called M-theory.