A Kekule struc-ture K of a hexagonal system H is also a perfect matching (1-factor) of H. A cycle (or circuit) C in H is said to be conjugated or resonant if there is a Kekule structure K of H such that C is a K-alternating cycle.

• 六角系统H的一个完美匹配(1-因子)也叫做H的一个凯库勒结构。 六角系统H中的圈C称为共振的，如果存在一个H的凯库勒结构K使得C是K-交错圈。