... In this poster we propose a hypothesis that within fixed assumptions about internal physics (opacity, thermodynamics, nuclear reactions) a strict relation between the surface helium abundance and the entropy of the adiabatic part of the convective zone exists for all solar models. This hypothesis is verified with models under simplified physics (Christensen­Dalsgaard 1988) and with six evolutionary models computed by Christensen­Dalsgaard (1992). ... 1990, Christensen­Dalsgaard 1991). ...