In main-sequence stars heavier than 9 Al , convective instability develops just outside the zero-age convective core as hydrogen depletion proceeds. At least ten schemes are available in the literature to treat the unstable intermediate zone which results. A detailed investigation is presented here, utilizing physical, laboratory, stellar-model, and observational considerations. Only two schemes survive, both of which are semiconvective: the modified Schwarzschild- (SH) piescription and the modified Ledoux-Sakashita- Hayashi (LSH) prescription. The former scheme is tentatively favoured by the weight of laboratory evidence. In either case, the earliest stages of hydrogen depletion are characterized by a semiconvective zone which joins continuously the convective core and the outer radiative envelope. The later stages are characterized by a radiative detachment of the semiconvective zone from the core, except for masses above 15 Al0 in the SH scheme. At any given mass, the final hydrogen profiles resulting from each of the two schemes are very similar.