  Let $A$ be a sheaf of Azumaya algebras over a Noetherian base $S$. In this
paper we describe using generalized Severi-Brauer varieties, a quasi-projective
moduli space parametrizing sheaves of \`etale subalgebras of $A$.
  In the case that $S$ is the spectrum of a field, we study the geometry of
this moduli space, and show that in certain cases it is a rational variety.
