The line graph $Γ$ of a multi-graph $Δ$ is the graph whose vertices are the edges of $Δ$, where two such edges are adjacent if and only if they meet in a single vertex of $Δ$. We provide several characterizations of such line graphs and in particular show that a graph is a line graph if and only if it does not contain one of $33$ graphs, all of which correspond to bases of anisotropic vectors of a $6$-dimensional orthogonal geometry of $-$-type over a field with two elements, or, equivalently, to sets of $6$ generating reflections in the Weyl group of type $E_6$.