The most obvious way to define a graph is a structure with a signature σ consisting of a single binary relation symbol "E". The vertices of the graph form the domain of the structure, and for two vertices "a" and "b", formula_58  means that "a" and "b" are connected by an edge. In this encoding, the notion of induced substructure is more restrictive than the notion of subgraph. For example, let "G" be a graph consisting of two vertices connected by an edge, and let "H" be the graph consisting of the same vertices but no edges. "H" is a subgraph of "G", but not an induced substructure. The notion in graph theory that corresponds to induced substructures is that of induced subgraphs.