Figure shows several representations of the same 2-periodic graph. The graph is made of two kinds of vertices (represented by open and closed circles), but the choice of the vertex identifiers and of the cell influences the representation. Each choice of cell is characterised by its origin and its axes x and y; some repeated cells are represented as well, along with their offset in the bottom-left corner. For each representation, the key is written below, each line referring to a direct edge. Recall that the Systre algorithm works by choosing the lexicographically smallest key from a set of representations, extracted from candidates that do not depend on the representation of the graph. Hence, if the set of representations was those shown in the figure, the topological genome of the graph would be the smallest key, i.e. that under representation (a).