One of the most important conventions of heraldry is the so-called "rule of tincture". To provide for contrast and visibility, metals should never be placed on metals, and colours should never be placed on colours. This rule does not apply to charges which cross a division of the field, which is partly metal and partly colour; nor, strictly speaking, does it prevent a field from consisting of two metals or two colours, although this is unusual. Furs are considered amphibious, and neither metal nor colour; but in practice ermine and erminois are usually treated as metals, while ermines and pean are treated as colours. This rule is strictly adhered to in British armory, with only rare exceptions; although generally observed in continental heraldry, it is not adhered to quite as strictly. Arms which violate this rule are sometimes known as "puzzle arms", of which the most famous example is the arms of the Kingdom of Jerusalem, consisting of gold crosses on a silver field.