  We elaborate on the recently discovered spinor-vector duality in realistic
free fermionic heterotic vacua. We emphasize the interpretation of the
freely-acting orbifolds carried out on the six internal dimensions as
coordinate-dependent compactifications; they play a central role in the
duality, especially because of their ability to break the right-moving
superconformal algebra of the space-time supersymmetric heterotic vacua. These
considerations lead to a simple and intuitive proof of the spinor-vector
duality, and to the formulation of explicit rules to find the dual of a given
model. We discuss the interest of such a duality, notably concerning the
structure of the space of vacua of superstring theory.
