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The most basic rule of heraldic design is the rule of tincture: metal should not be put on metal, nor colour on colour. This means that the heraldic metals or and argent should not be placed on each other, nor may any of the colours be placed on another colour. Heraldic furs as well as "proper" are exempt from the rule of tincture.
The rule seems to have operated from the inception of the age of heraldry, i.e. about 1200–1215, but seemingly was never written down. It was rather deduced by later commenters as a rule which must have existed, based on the evidence it produced. Although the vast majority of coats of arms ever used across the whole of Europe follow the rule, a very few coats which contravened the rule were borne in the mediaeval era by certain families or corporate bodies for many centuries without effective censure by the heraldic authorities. The reason for the original contraventions and for the toleration of them is unknown, although in the case of the arms of the Kingdom of Jerusalem, clearly extremely high status was involved.