  The discrepancy in the past years of many more black-hole soft X-ray
transients (SXTs), of which a dozen have now been identified, had challenged
accepted wisdom in black hole evolution. The sheer number of inferred SXTs
requires that many, if not most, stars of ZAMS masses $20-35\msun$ end up as
black holes (Portegies Zwart et al. 1997; Ergma & van den Heuvel 1998). In this
paper we show that this can be understood by challenging the accepted wisdom
that the result of helium core burning in a massive star is independent of
whether the core is covered by a hydrogen envelope, or "naked" while it burns.
The latter case occurs in binaries when the envelope of the more massive star
is transferred to the companion by Roche Lobe overflow while in either main
sequence or red giant stage. For solar metallicity, whereas the helium cores
which burn while naked essentially never go into high-mass black holes, those
that burn while clothed do so, beginning at ZAMS mass $\sim 20\msun$, the
precise mass depending on the $^{12}C(\alpha,\gamma)^{16}$O rate as we outline.
In this way the SXTs can be evolved, provided that the H envelope of the
massive star is removed only following the He core burning. We also show that
in order to evolve a black hole of mass $\gsim 10\msun$ such as observed in Cyg
X-1, even employing extremely massive progenitors of ZAMS mass $\gsim 60\msun$
for the black hole, the core must be covered by hydrogen during a substantial
fraction of the core burning. In other words, the progenitor must be a WNL
star. We evolve Cyg X-1 in an analogous way to which the SXTs are evolved, the
difference being that the companion in Cyg X-1 is more massive than those in
the SXTs, so that Cyg X-1 shines continuously.
