Plate armour gave better protection than mail. By about 1300 new types of arrow and thrusting swords had been developed that could pierce mail armour. This led to the development of plate armour, made up of sheets of steel shaped to fit over the arms, legs or body. Arrows and swords tended to glance off plate armour without penetrating.
from "100 FActs about Arms and Armour" by Rupert Matthews.