TITLE: Graph on the cover of Bollobás's "Combinatorics"
QUESTION [9 upvotes]: I was browsing the library and I found Bela Bollobás book "Combinatorics: Set Systems, Hypergraphs, Families of Vectors and Probabilistic Combinatorics" and, on its cover, it has a graph that I don't quite recognize:

Is this graph any "interesting", as used for counterexamples (e.g., Petersen's Graph)? I could determine some of its characteristics (9 vertices, regular of valency 4, non-planar, diameter 2 etc.), but is it distinguished in any way for being featured in an important book?

REPLY [2 votes]: It's the unit distance embedding of $K_3 \times K_3$, one of the few strongly regular graphs that has a unit distance embedding in $\mathbb{R}^2$. It is also known as $3 \times 3$ rook graph (though sometimes mistakenly as $3 \times 3$ grid graph). It also is a generalized quadrangle as pointed out above by A. Schulz.