The quark-line rule has been reviewed both experimentally and theoretically. The present experimental data are consistent with the good validity of the rule for the 1− and possibly 2+ nonets. The rule is also in reasonable agreement with the 0− nonet. The possible violation of the rule has been estimated to be 6% for the 1− nonet and 15% for the 0− nonet. So far the best explanation of the quark-line rule is perhaps by quantum chromo-dynamics, although the existence of an unknown symmetry group responsible for it is not ruled out.