 At the intersection of these strategies, Bigelow and Pargetter (1989, 1991) claim that structural universals are internally relational properties that supervene on first-order properties and relations and on second-order proportion relations. Second-order proportion relations are relations such as having four times as. They relate first-order properties and relations. For example, in methane, the relation having four times as relates two conjunctive properties: (i) hydrogen and being part of this molecule; (ii) carbon and being part of this molecule.