The first rule of heraldic design is the rule of tincture: metal should not be put on metal, nor colour on colour (Humphrey Llwyd, 1568). This means that Or and argent (gold and silver, which are represented by yellow and white) may not be placed on each other; nor may any of the colours (i.e. azure, gules, sable, vert and purpure) be placed on another colour. Heraldic furs (i.e. ermine, vair and their variants) as well as "proper" (a charge coloured as it normally is in nature) are exceptions to the rule of tincture.