Which came first to the designers of chess: the rules or the metaphor? It feels impossible to prise the system from the story: a military battle between two monarchs, each with perfectly symmetrical assets and equally balanced capabilities. Yet there have been dozens of ‘reskins’ of chess, swapping the kings and their minions for characters from, say, Lord of the Rings, or The Simpsons, or even, bewilderingly, M&M chocolates.