  We argue that the projective special linear group PSL(2,7), also known as
Sigma(168), has unique features which make it the most suitable discrete family
symmetry for describing quark and lepton masses and mixing in the framework of
SO(10) type unified models. In such models flavon fields in the sextet
representation of PSL(2,7) play a crucial role both in obtaining tri-bimaximal
neutrino mixing as well as in generating the third family charged fermion
Yukawa couplings. In preparation for physical applications, we derive the
triplet representation of PSL(2,7) in the basis S,T,U,V where S,T,U are the
familiar triplet generators of S(4) in the diagonal charged lepton basis where
T is diagonal. We also derive an analogous basis for the real sextet
representation and identify the vacuum alignments which lead to tri-bimaximal
neutrino mixing and large third family charged fermion Yukawa couplings.
