Plot of the density-sensitive [S II] ratio as a function of diameter for all the SNRs, with different symbols for those that have been detected in X-rays (red squares) and those that have not (blue triangles; X-ray data from L14). Emission from smaller remnants generally stems from higher-density material, suggesting that the smallest (and likely youngest) are evolving into dense circumstellar material, possibly material shed by the precursor stars. Furthermore, the X-ray detected remnants generally have higher densities, and are smaller (on average) than the others.