The issue I want to discuss is this: supernovae observed during their photospheric phases display three distinct kinds of spectra - but there are four distinct chemical compositions that should be considered for the outer layers of exploding stars. What is the correspondence between the observed supernova type and the chemical composition?The three kinds of observed spectra are Type II, which by definition shows optical hydrogen lines, Type Ia, which has neither hydrogen nor helium lines, and Type Ib, which has helium but not hydrogen.The compositions to be considered are hydrogen-rich, or, speaking loosely, “solar”; helium-rich, or, loosely, “Wolf-Rayet”; deflagration - initially a mixture of heavy elements from carbon to radioactive nickel, which decays through cobalt to iron; and detonation - initially a mixture of just helium and radioactive nickel.