We introduce the notion of semi-Cartan C*-subalgebras, which are commutative subalgebras that, unlike Cartan subalgebras, are not required to be maximal abelian. Despite this relaxation, they retain enough structure to fully characterize a broad class of C*-algebras arising from groupoids. Specifically, we show that a C*-algebra contains a semi-Cartan subalgebra if and only if it is isomorphic to a twisted étale groupoid C*-algebra, possibly nonreduced and associated to a noneffective groupoid. This generalizes the Kumjian–Renault framework.This is joint work with Tristan Bice, Ying-Fen Lin, and Kathryn McCormick.
