Baez refers to the Brane Scan in a old TWF, #118. You can also find a more recent depiction in http://arxiv.org/abs/hep-th/0301037 Duff also refers to original work from German Sierra (1987) and from J.M. Evans (1988, KEK scan avail.; also see hep-th/9410239 for updated bibliography). Perhaps see also Kugo & Townsend. Of course when you see the set 0,1,3,7 two mathematical neurons trigger in your brain. The topological one mutters "comb-able" spheres, the algebraist shouts "division algebras". According Duff, the 1984 Green Schwarz paper already states the result of the existence of only four classical supersymmetrical string theories, in st dimensions 10, 6, 4 and 3. What is new in the Brane Scan is that it shows "ladders", Code (Text): D 11 O X ? 10 o O o o o H o o o o 9 O H 8 O H 7 H X 6 h H h C h h 5 H C 4 H c C R c 3 C R r 2 C R 1 R -1 0 1 2 3 4 5 6 7 8 9 10 p so it seems that there are two different ways for the division algebras to appear in string theory, only that they coincide in the 1-brane case. Or two different ways to produce branes, and both depending of the existence of superstrings. The horizontal way is related to Vector multiplets, the diagonal way is related to Scalar multiplets. The two extant "X" relate to tensor multiplets. Not really that I understand what does it mean, I am just copying from the table.