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Let G be a finite bipartite plane graph. In a chemical context, a set of pairwise disjoint edges that cover all vertices of G (i.e., a perfect matching of G) is called a Kekulé pattern of G, and a set of pairwise disjoint cells of G such that the deletion of all vertices incident to these cells results in a graph that has a Kekulé pattern, or is empty, is called a Ciar pattern of G. Let k(G) and c(G) denote the number of Kekulé patterns and of Ciar patterns of G, respectively. It is shown that k(G) is not smaller than c(G) and that equality holds if G is an outerplane graph. This result generalizes a well-known proposition of the theory of benzenoid hydrocarbons; the proof uses a new idea.

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