  A three-generation Pati-Salam model is constructed by compactifying the
heterotic string on a particular T^6/Z_6 Abelian symmetric orbifold with two
discrete Wilson lines. The compactified space is taken to be the Lie algebra
lattice G_2+SU(3)+SO(4). When one dimension of the SO(4) lattice is large
compared to the string scale, this model reproduces many features of a 5d
SO(10) grand unified theory compactified on an S^1/Z_2 orbifold. (Of course,
with two large extra dimensions we can obtain a 6d SO(10) grand unified
theory.) We identify the orbifold parities and other ingredients of the
orbifold grand unified theories in the string model. Our construction provides
a UV completion of orbifold grand unified theories, and gives new insights into
both field theoretical and string theoretical constructions.
