  The structure of a certain subgroup $S$ of the automorphism group of a
partially commutative group (RAAG) $G$ is described in detail: namely the
subgroup generated by inversions and elementary transvections. We define
admissible subsets of the generators of $G$, and show that $S$ is the subgroup
of automorphisms which fix all subgroups $\langle Y\rangle$ of $G$, for all
admissible subsets $Y$. A decomposition of $S$ as an iterated tower of
semi-direct products in given and the structure of the factors of this
decomposition described. The construction allows a presentation of $S$ to be
computed, from the commutation graph of $G$.
