Illustration of the helium content of massive star progenitors at core collapse and evolved either in isolation or in an interacting binary system. For single-star progenitors, we use the models of Sukhbold et al. (2016), labeled S16, and show the combined masses of the He/C and the He/N shells in the progenitor versus the helium-core mass, both taken at the time of core collapse. For the binary star progenitors, we use the helium-star models of Woosley (2019), labeled W19 (we include models evolved with three different mass loss rates), and show the total helium yields versus pre-SN mass. The color coding indicates the ZAMS mass for each model. (See Sect. 2 for discussion).