A rule that determines whether the lowest excited triplet state of a planar cyclic molecule is aromatic. It is the reverse of the Hückel rule (see aromatic compound), as it states that the lowest excited state for such molecules has 4n pi-electrons for aromaticity, where n is a positive integer, and (4n + 2) pi-electrons for pseudo-aromaticity. The rule was stated by the Canadian chemist Colin Baird in 1972 on the basis of quantum mechanics.