From J.J. Zakurai page no.222-225: We know that Clebsch-Gordan coefficients vanish unless m=m1+m2. Then, (1) why is that the terms in recursion relation doesn't vanish? since m1+m2=m+1 or m1+m2=m-1 in eqn.(3.7.49). (2) why is again in the example shown in eqn.(3.7.54), m1+m2=m? I couldn't understand the whole thing. Somebody please help.