On exit from nauty, lab[i] is the OLD name of the NEW vertex numbered i. So 0 3 1 2 4 5 means new 0 = old 0 new 1 = old 3 new 2 = old 1 new 3 = old 2 new 4 = old 4 new 5 = old 5 So the relabelling function mapping vertices to their new names is the inverse of the permutation written immediately before the graph. Gordon