Definition 4

Let G be a graph and $x$ an object. The set $G\langle \{x\} \rangle$ (which is sometimes denoted by $G(x)$, by abuse of language) is called the section of G at $x$.

It follows immediately from C43 (Chapter I, § 5, no. 1) that the relation $y \in G\langle \{x\} \rangle$ is equivalent to $(x, y) \in G$. If G and $G'$ are two graphs, the relation $G \subset G'$ is thus equivalent to

If $\Gamma = (\mathrm{G}, \mathrm{A}, \mathrm{B})$ is a correspondence between A and B, then for every $x \in \mathrm{A}$ the section of G at $x$ is also called the section of $\Gamma$ at $x$ and is denoted by $\Gamma\langle\{x\}\rangle$ (or $\Gamma(x)$).