The observables T/S and n-1 are key to testing and understanding inflation.
(T, S, and n-1 respectively quantify the gravity-wave and density-perturba
tion contributions to CMB anisotropy and the deviation of the density pertu
rbations from the scale-invariant form.) Absent a standard model, there is
no definite prediction for, or relation between, T/S and n-1. By reformulat
ing the equations for slow-roll inflation, we show that in the T/S-(n-1) pl
ane there are excluded regions, regions in which the density perturbations
are not well approximated by a power law, and regions in which models with
a "featureless" potential must lie.