<imgclass="img-responsive"alt="graph unit A"src="https://cdn.freecodecamp.org/curriculum/project-euler/coloured-configurations-1.png"style="display: inline-block; background-color: white; padding: 10px;">
and B: <imgclass="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 <imgclass="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 $c ≥ 4$