A supposedly immutable rule of heraldry is that gold (Or) and silver (Argent) should never be placed side-by-side or one upon the other. Heim, the Papal Nuncio who designed four popes’ coats of arms, investigates when and where this rule originated and how it has been observed in different countries. But he also shows how often the rule is broken, identifying more than 360 such coats of arms from across Europe, including his own.