Which areas of the body still required mail after full plate armour was developed?

Even after the development of full plate armour, mail remained a vital component of a knight's defensive equipment. Rather than being entirely replaced, strips of mail were worn underneath the rigid plate armour to protect areas where gaps existed between the metal plates.1

These gaps typically occurred at the joints and other points of articulation, where the rigid steel plates could not provide full coverage without restricting the wearer's movement. By wearing mail in these vulnerable areas, armourers ensured that the knight remained protected against thrusting weapons that might otherwise penetrate the joints or seams of the plate suit.1