 Traditional logic studied certain simple relationships between classes, namely being a subclass of and having a nonempty intersection with. However, once one adopted an axiomatic approach, the topic of possible models besides the obvious ones surfaced. Beltrami introduced models of non-Euclidean geometry in the late 1860s. In the 1890s Schröder and Dedekind constructed models of the axioms of lattice theory to show that the distributive law did not follow. But when it came to the algebra of classes, Schröder considered only the standard models, namely each was the collection of all subclasses of a given class.