Given E-0, E-1, E, F rearrangement invariant spaces, a, b, b(0), b(1) slowly varying functions and 0 <= theta(0) < theta(1) <= 1, we characterize the interpolation spaces (<(X)over bar>(theta)(0) (,b0,)(E0),(X) over bar (R)(theta 1)(,)(b1)(,)(E1)(,a,F))(theta,b,E) and ((X) over bar (L)(theta 0,)(b0)(,)(E0)(,a,F), (X) over bar (theta 1,)(b1)(,)(E1))(theta,b,E), for all possible values of theta is an element of [0,1]. Applications to interpolation identities for grand and small Lebesgue spaces, Gamma spaces and A and B-type spaces are given.