Evolution of the C-burning front, the C-free core (which follows the progression of neon burning), and the iron-rich core as a function of time from carbon burning to core collapse for models before and up to the compactness peak (region A of Fig. 1, see inset). As carbon burning becomes increasingly neutrino-dominated, the C-burning front moves further out in mass, ultimately leading to the formation of a larger C-free and iron-rich core.