and B: <img class="img-responsive" alt="graph unit B" src="https://cdn.freecodecamp.org/curriculum/project-euler/coloured-configurations-2.png" style="display: inline-block; background-color: white; padding: 10px;">, where the units are glued along the vertical edges as in the graph <img class="img-responsive" alt="graph with four units glued along the vertical edges" src="https://cdn.freecodecamp.org/curriculum/project-euler/coloured-configurations-3.png" style="display: inline-block; background-color: white; padding: 10px;">.
A configuration of type $(a,b,c)$ is a graph thus built of $a$ units A and $b$ units B, where the graph's vertices are coloured using up to $c$ colours, so that no two adjacent vertices have the same colour. The compound graph above is an example of a configuration of type $(2,2,6)$, in fact of type $(2,2,c)$ for all $c ≥ 4$