Figure 1.

(Top) Two 2-periodic graphs defined on the same infinite graph G and differing by the periodicity group Γ acting on G. Generators of the two lattices and the corresponding unit cells are highlighted, and orbits of vertices and edges are similarly coloured. (Left) For maximal periodicity, there are two vertex orbits and three edge orbits. (Right) With an index two sublattice, there are four vertex orbits and six edge orbits. (Bottom) The corresponding quotient graphs, with matching vertex and edge colours, together with a schematic for the placement of vertex and edge representatives. (Online version in colour.)