M Theory

The understanding of how the IIA and HE theories behave at strong coupling, which is by now well-established, came as quite a surprise.
In each of these cases there is an 11th dimension that becomes large at strong coupling, the scaling law being

In the IIA case the 11th dimension is a circle, whereas in the HE case it is a line interval (so that the eleven-dimensional space-time has two ten-dimensional boundaries).
The strong coupling limit of either of these theories gives an 11-dimensional space-time. The eleven-dimensional description of the underlying theory is called "M theory." As yet, it is less well understood than the five 10-dimensional string theories.
The connections among theories that we've mentioned are sketched below.