This means that we will need matrix elements of the type $$ \langle Y^{l'}||Y^{\lambda}|| Y^{l}\rangle. $$ We can rewrite the Wigner-Eckart theorem as $$ \langle Y^{l'}||Y^{\lambda}|| Y^{l}\rangle=\sum_{m\mu}\langle \lambda\mu lm|l'm'\rangle Y^{\lambda}_{\mu}Y^l_m, $$ This equation is true for all values of \( \theta \) and \( \phi \). It must also hold for \( \theta=0 \).