# Symmetry relation of Wigner-Eckart

I saw a symmetry relation following from the the Wigner-Eckart Theorem looking like this

$$(\xi j|| T_L || \xi'j') = (-1)^{j-j'} (\xi' j'|| T_L || \xi j)^*$$

I know that it must come somehow under hermitian conjugation.

But I really don't know how to obtain that prefactor $$(-1)^{j-j'}$$ .