M-theory
The combination is accomplished by knitting together a web of relationships between each of the string theories called dualities (specifically, S-duality, T-duality, and U-duality). Each of these dualities provides a way of converting one of the string theories into another. T-duality is probably the most easily explained of the dualities. It has to do with the size, written as R, of the curled up dimensions of the string theories. It was discovered that by taking a Type IIA string theory that has a size R and changing the radius to 1/R the result will end up being what is equivalent to a Type IIB theory of size R. This duality, along with the others, creates connections between all 5 (or 6, if supergravity is counted) theories. The fact that these dualities existed had been known before Witten came up with the idea of M-theory.