Before string theory was well-known in the 70s and early 80s, physicists were working on supergravity, a supersymmetric quantum field theory that incorporated general relativity. The best supergravity theory was eleven-dimensional, but it had its problems. In 1995, Witten ressurected it to demonstrate its applicability to string theory. He argued that as the string coupling constant is increased from less than one to much greater than one in Type IIA theory, it is a low-energy approximation of eleven-dimensional supergravity. This connected string theory and supergravity in the following way: as the coupling constant is increased, another dimension becomes visible and the strings gain a second dimension. In the case of Heterotic-E theory, strings gain height and become a membrane. In type IIA theory, the strings become two-dimensional donut-like objects.

But it goes beyond two dimensions. There are also three, four, five, all the way up to nine-dimensional shapes. These are called three-branes, four-branes, five-branes, and so on using the pattern of P being a whole number of dimensions with the shape taking the form P-brane. Isn't that cute?

In the diagram you can see how the various pieces of string theory are dual-related. There are other links between Heterotic-O theory and Heterotic-E, as well as between Type IIA and Type IIB. In a Heterotic-O universe with a circular dimension of radius R, the physics are the same as for a Heterotic-E universe with radius 1/R. Something similar holds true with Type IIA and Type IIB. Everything is interrelated in the new eleven-dimensional theory coined M-theory.