The arm harness is the assembly of plates covering shoulder, upper arm, elbow and forearm — pauldron, rerebrace, couter and vambrace.
An arm harness is not one object but an assembly, named by position: a pauldron or spaulder over the shoulder, a rerebrace on the upper arm, a couter at the elbow, and a vambrace on the forearm.
The problem it solves is the hardest in armour after the hand. The arm has two major joints, it must move through a very wide range, and it is held out in front of the body where it takes blows before anything else does.
Its solution is articulation: rigid plates over the long bones, overlapping lames across the joints, and mail filling what plate cannot reach.
The rerebrace and vambrace are tubes, each usually made in two halves hinged along one side and closed with a strap, so they can be sprung open to get the arm in and then shut around it.
The couter spans the elbow with a shaped cop, and its distinguishing feature is the fan or wing standing out on the outer side, covering the joint when the arm bends and shielding the inside of the elbow from a thrust.
The shoulder is either a spaulder — narrow, following the shoulder line — or a pauldron, which is larger and wraps over the shoulder onto the chest and back. Where the pauldron leaves a gap at the armpit, a besagew, a small disc on a strap, hangs across it.
The arms are the first thing an opponent can reach, and the fight books say so. A cut to an unarmoured arm ends a fight as decisively as one to the head, and any harness that leaves the arms bare is not a harness.
The armpit is the real prize. It cannot be covered in rigid plate without stopping the arm moving, so it is filled with mail and shielded by the besagew, and the fifteenth-century armoured-combat manuals send the point there again and again.
The whole assembly also has to permit half-swording, poleaxe work and grappling, which is a demanding requirement: the wearer must be able to bring both hands together in front of his chest and above his head without a gap opening anywhere.
Its strength is that it covers a moving limb completely. A well-made fifteenth-century arm harness has no position within the arm's natural range in which bare flesh is exposed.
Its limitation is the trade against reach and weight. Every lame added to a joint improves coverage and costs a little movement, and the arms are where a wearer notices added weight most because he is lifting it constantly.
The joints are also where a harness fails first. Straps wear, leathers rot and rivets shear, and loose arm defences shift out of position at exactly the moment they are needed.
Through the thirteenth century the arm is protected by the sleeve of a mail hauberk and nothing else. From the early fourteenth, plate cops are strapped over the mail at the elbow — the first plate on the arm anywhere.
Splinted defences and hardened leather follow, and by about 1400 the full plate arm exists: shoulder, upper arm, elbow and forearm all in articulated steel, with mail surviving only at the armpit.
The fifteenth century refines articulation rather than adding coverage, and the sixteenth adds bulk and decoration before the whole assembly is abandoned as firearms make it not worth its weight.
Italian arm harness is characteristically asymmetric. The left arm faces the opponent in the lists and in the charge, so its pauldron is made larger and heavier, while the right is cut away to let the lance and sword arm work.
German gothic arms are narrower, more sharply pointed and fluted in line with the rest of the harness, and German practice tends to prefer spaulders where Italian prefers the wrap-around pauldron.
Both traditions converge on the same underlying answer, and the differences are in emphasis rather than in mechanism.