  The heterotic-string models in the free fermionic formulation are among the
most realistic string vacua constructed to date, which motivates their detailed
exploration. Classification of free fermionic heterotic-string vacua revealed a
duality under exchange of spinor and vector representations of the SO(10) GUT
symmetry. The spinor-vector duality was subsequently demonstrated in a Z2 X Z2
X Z2 orbifold, in which the map is realised as exchange of discrete torsions.
Analysis of the orbifold partition function shows that the duality map
preserves the number of massless twisted states. While the dual vacua are
distinct from the point of view of the low energy field theory it is suggested
that they are equivalent from the string point of view and may be connected by
continuous and discrete transformations.
