  Tri/bi-maximal mixing (TBM) is a specific lepton mixing ansatz, which
describes the trend of the current neutrino oscillation data, in particular the
recent SNO and KAMLAND results. The significant feature of TBM in this respect
is |U_e2|^2=|U_m2|^2=|U_t2|^2=1/3, and we say that the nu_2 is tri-maximally
mixed. We have generalised the TBM ansatz to a generic mixing matrix with the
nu_2 trimaximally mixed, whereby the neutrino mass matrix in the lepton flavour
basis takes the form of a general S3 group matrix (3 x 3 `magic-square'). In
exact TBM the charged-lepton mass matrix in the neutrino mass basis (where the
neutrino mass matrix is diagonal) takes the form of a general S3 class
operator. The neutrino mass matrix in the flavour basis is a particular S3
group matrix which is also an S1 C S2 C S3 group-chain class operator, whereby
the neutrino mass eigenstates are distinguished by their `mutativity' (M_i =
+/-1) and `democracy' (D_i = 0,3) which are both good quantum numbers in exact
TBM.
