How quantum phases on cylinders approach the 2d limit

We consider the properties of $T=0$ quantum phases of matter - especially superconducting and analogous spin-liquid phases - on infinite cylinders of width $L_\perp$ and analyze the ways in which the $L_\perp \to \infty$ (2d) limit is approached. This problem is interesting in its own right, but is particularly important in the context of extrapolating accessible density matrix renormalization group (DMRG) results on model strongly interacting problems to the desired 2d limit. Various methods for drawing firm conclusions about the quantum phases in 2d from relatively small $L_{\perp}$ results are illustrated.