  We present a comprehensive description of the Sch\"onberg--Chandrasekhar
(S--C) transition, which is an acceleration of the stellar evolution from the
nuclear to the thermal time scales occurring when the fractional mass of the
helium core reaches a critical value, about 0.1. It occurs in the 1.4 to 7
M$_\odot$ mass range due to impossibility of maintaining the thermal
equilibrium after the nuclear energy sources in the core disappear. We present
the distributions of the hydrogen abundance, the energy generation rate and the
temperature for stars crossing that limit. We confirm that a sharp S--C limit
is present for strictly isothermal cores, but it is much smoother for real
stars. The way the boundary of the core is defined is important for the picture
of this transition. With a strict definition of the core as the region where
the helium abundance is close to null, it occurs in an extended range of the
fractional core mass of roughly 0.03 to 0.11. The cause of that is a gradual
core contraction causing a correspondingly gradual loss of the core
isothermality with the increasing core mass. On the other hand, when using
definitions allowing for some H abundance in the core, the S--C transition is
found to be sharper, at the fractional core mass of between about 0.07 and
0.11. Still, it is more a smooth transition than a sharp limit. We have also
searched for specific signatures of that transition, and found that it is
associated with the stellar radius first decreasing and then increasing again.
We have considered whether the S--C limit can be used as a diagnostic
constraining the evolutionary status of accreting X-ray binaries, but found
such uses unfounded.
