 In the first half of the nineteenth century there was no general agreement about how the notion of molecular structure could be deployed in understanding isomerism. But during the second half of the century, consensus built around the structural theories of August Kekulé (1829–1896). Kekulé noted that carbon tended to combine with univalent elements in a 1:4 ratio. He argued that this was because each carbon atom could form bonds to four other atoms, even other carbon atoms (1858 [1963], 127). In later papers, Kekulé dealt with apparent exceptions to carbon’s valency of four by introducing the concept of double bonds between carbon atoms. He extended his treatment to aromatic compounds, producing the famous hexagonal structure for benzene (see Rocke 2010), although this was to create a lasting problem for the universality of carbon’s valency of 4 (Brush 1999a, 1999b).