The difference between the two groups gives them slightly different masses, from the spin‑orbit coupling rule. Theoretically, the h and b mesons are in the 1 group, and should have heavier masses, according to the spin-orbit mass splitting. However, the measured masses of the mesons do not appear to follow the rule, as evidenced by the f and a mesons being heavier. There are considerable uncertainties in experimental measurement of pseudovector mesons; more experimental data will be needed to confirm and accurately determine the discrepancy between theory and measurement.