Membrane tension and manifest IIB S-duality

A manifestly S-dual, and ‘12 dimensional’, IIB superstring action with an Sl(2; R) doublet of ‘Born-Infeld’ fields is presented. The M-theory origin of the 12th dimension is the M-2-brane tension, which can be regarded as the flux of a 3-form worldvolume field strength. The latter is required by the fact that the M-2-brane can have a boundary on an M-5-brane.