Barding is armour for the warhorse — mail, quilted textile and hardened leather in the medieval centuries, with an iron shaffron over the head.
Barding is armour for the horse. In the medieval centuries it means a mail trapper, a quilted textile caparison, hardened leather pieces, and an iron shaffron over the head — not the articulated plate bard that most people picture.
It exists because of a simple asymmetry. A knight in full harness is very hard to kill; his horse, standing much larger and covered in nothing, is not. Killing the horse takes the man out of the fight just as effectively and is far easier.
It was extremely expensive and correspondingly rare. A mail trapper contains more metal than a man's hauberk, and only the wealthiest could field a barded horse — which is why it appears far more often on seals and effigies, where display mattered, than it can have done in the field.
The shaffron is the one component that survives in numbers. It is an iron plate shaped to the horse's forehead and muzzle, with raised cup-shaped guards over the eyes and simple straps, and iron shaffrons are attested from the thirteenth century.
The trapper is a mail covering hung over the horse's neck, chest and body, reaching to about the knees and split front and back so the legs move. It is made exactly like a hauberk and is enormously heavy — plausibly twenty to forty kilograms.
The caparison is textile: a fitted cloth cover, often quilted, worn over the mail or instead of it. It carried the owner's arms, which is the reason it appears in almost every heraldic image of a mounted knight and why so many people assume horses were routinely armoured.
Its job is to get the horse through the approach. A charge that arrives with its horses dead or bolting has failed before contact, and every missile weapon on a medieval battlefield was more likely to hit a horse than the man on it.
Against archery the effect is real but partial. Mail and quilted cloth will stop or slow many arrows, and even a shaffron that merely prevents a horse being blinded is doing decisive work — a wounded horse that keeps going is still a functioning cavalry mount.
The English tactical system of the fourteenth century is the argument for it. At the Battle of Crécy in 1346 and the Battle of Agincourt in 1415 the massed shooting did most of its work on horses and on unarmoured limbs, and the French response over that period includes both dismounting the men-at-arms and armouring the horses of those who stayed mounted.
Its strength is that it protects the part of the system most likely to fail. Nothing else a knight could buy addressed the horse, and the horse was both the expensive component and the vulnerable one.
Its limitations are weight and heat. A mail trapper of thirty kilograms on top of saddle, rider and the rider's own harness is a serious load, and it costs the horse speed and stamina in exactly the moments that matter.
Cost is the real constraint. Barding roughly doubles the price of equipping a man-at-arms, and the documentary record makes clear that most mounted men rode unarmoured horses however much the art suggests otherwise.
Textile caparisons appear from the twelfth century, initially as much for display and weather as for defence, and become the standard visual attribute of the mounted knight in seals and effigies.
Mail trappers follow in the later twelfth and thirteenth centuries for those who could afford them, and iron shaffrons are documented from the thirteenth — the head being both the most vulnerable point and the cheapest to armour.
Plate comes late. Articulated bards with a crinet for the neck, a peytral for the chest and a crupper for the hindquarters develop only in the second half of the fifteenth century, and the great majority of complete surviving bards are sixteenth-century objects belonging to the period after this archive.
Western European practice is the best documented, through English, French and Burgundian inventories and through the effigies and brasses that show caparisons in heraldic detail.
The Iberian frontier went the other way. Light cavalry fighting rewarded speed over protection, and the jinete tradition rode unarmoured horses deliberately rather than for want of means.
Eastern and steppe practice shows lamellar and leather horse armour on quite different principles, encountered directly during the Mongol invasions and known from finds well outside Latin Europe.