  Modelling the base of the solar convective envelope is a tedious problem.
Since the first rotation inversions, solar modellers are confronted with the
fact that a region of very limited extent has an enormous physical impact on
the Sun. Indeed, it is the transition region from differential to solid body
rotation, the tachocline, which furthermore is influenced by turbulence and is
also supposed to be the seat of the solar magnetic dynamo. Moreover, solar
models show significant disagreement with the sound speed profile in this
region. In this paper, we show how helioseismology can provide further
constraints on this region by carrying out an inversion of the Ledoux
discriminant. We compare these inversions for Standard Solar Models built using
various opacity tables and chemical abundances and discuss the origins of the
discrepancies between Solar Models and the Sun.
