The medieval knight’s mail shirt, worn beneath plate armor, was called a hauberk.
A hauberk was a long shirt of interlinked metal rings, usually reaching to the knees and often including sleeves. Mail protected the torso, arms, and sometimes the upper legs while allowing more flexibility than a solid metal cuirass. Its construction required large numbers of individually formed and linked rings.
The term comes through Old French from a Germanic word associated with protection for the neck or body. In earlier periods, a hauberk could serve as a knight’s main body armor. As plate armor developed in the late Middle Ages, mail remained useful at vulnerable joints, beneath armor plates, and in areas requiring movement.
A hauberk is not the same as a gambeson, which was a padded textile garment. Nor is it identical to a haubergeon, a shorter form of mail armor. Modern descriptions also often use “chainmail,” although medieval sources more commonly referred to the material simply as mail.