The linked or mail armor now began to be superseded by that made of
pieces of steel joined together so as to fit the body. This, when it
was finally perfected, was called plate armor, and was both heavier
and stronger than mail.
With the introduction of plate armor and the closed helmet it became
the custom for each knight to wear a device, called a crest, on his
helmet, and also to have one called a coat of arms (because originally
worn on a loose coat over the armor).
The coat of arms served to distinguish the wearer from the others, and
was of practical use not only to the followers of a great lord, who
thus knew him at a glance, but it served in time of battle to prevent
the confusion of friend and foe. Eventually, coats of arms became
hereditary, and the descent, and to some extent the history, of a
family can be traced by them. In this way heraldry may often prove
helpful in gaining knowledge of men and events.