Under a full suit of plate armor, a medieval knight wore several carefully designed layers: linen undergarments against the skin, a padded and fitted garment called an arming doublet over that, and often patches of mail sewn directly onto the doublet to protect gaps in the plates. Each layer served a specific purpose, from wicking sweat to absorbing blows to physically holding the steel plates in place.
Linen Base Layer: Braies and Shirt
The layer closest to the skin was simple linen. Knights wore braies, a loose-fitting undergarment similar to drawstring shorts, made from fine unbleached or white linen. On top, a linen shirt served as the first real barrier between skin and everything above it. Linen was the material of choice because it breathes well, absorbs moisture, and dries relatively quickly. Given that a full harness of plate could trap enormous amounts of body heat, this base layer mattered more than it might seem. Studies on body armor and heat stress show that covering the torso with rigid protection nearly triples the resistance to sweat evaporation compared to an uncovered body, which means anything worn underneath needed to pull moisture away from the skin as efficiently as possible.
The Arming Doublet
The arming doublet was the most important garment under plate armor. It was a close-fitting, lightly padded jacket that served as the foundation for the entire suit. Every other piece of armor ultimately connected to it or rested on it.
Arming doublets were typically made from multiple layers of linen, sometimes with flax stuffing between them. Italian examples from the 15th century used linen and silk for the lacing points, tipped with small metal caps called aiglets to keep the cords from fraying. The doublet was not meant to be bulky. Unlike a standalone gambeson (which could be worn as armor on its own), the arming doublet was deliberately thin, because it had to fit snugly beneath rigid steel. A doublet that was too thick would restrict movement and make the armor fit poorly. Its padding was just enough to cushion the wearer from the hard edges of the plates and soften the force of blows transmitted through the steel.
The doublet also shaped the wearer’s body into the silhouette the armor was built around. Plate armor was custom fitted, and the arming doublet helped create the correct profile so that each piece sat where it was designed to sit. This improved both comfort and protection, since plates that shift or gap leave the wearer vulnerable.
How It Held the Armor On
Steel plates were not simply strapped on over a passive layer of clothing. They were physically tied to the arming doublet using short cords called arming points. These were sturdy laces, historically made from linen cord or thin leather strips, threaded through small holes in both the doublet and the armor pieces, then knotted tight. The arm defenses, leg armor, and other components all attached this way. It was a time-consuming process (knights needed help getting armed), but it meant the weight of individual plates was transferred directly to the doublet and, through it, distributed across the torso and hips rather than hanging from a single point.
The durability of these cords was a real concern. Leather strips treated with oil could last years, but untreated leather or cheap cord could snap under stress. Linen cords with metal aiglet tips were the standard solution for most of the 15th century, balancing strength with the ability to tie secure knots that wouldn’t slip during combat.
Mail Sewn Onto the Doublet
Plate armor left certain areas exposed, particularly where the body needed to bend. The armpits, the inner elbows, and the groin were the most vulnerable spots. To cover these gaps, patches of chainmail called voiders or gussets were sewn directly onto the arming doublet. This was a practical, elegant solution: the mail moved with the wearer’s body and protected exactly the areas the rigid plates could not reach.
Armpit voiders were the most common, covering the underside of the arms where an opponent’s blade or point could slip between the chest plate and the arm harness. Groin protection in the form of a short mail skirt attached to the doublet’s lower edge appears in late 15th-century sources. The back of the legs, interestingly, was rarely covered this way. A well-known manuscript illustration from that period, the Hastings Manuscript, shows a knight being dressed in an arming doublet with clearly visible mail gussets and a mail skirt, giving a detailed look at how the complete system went together.
The Gambeson as an Alternative
Before the arming doublet became standard in the 15th century, and sometimes alongside it, knights wore a gambeson: a thicker, heavier padded jacket made from many layers of linen or wool stitched together. Gambesons ranged from 2 to 6 layers of padding. Under solid plate armor, 3 layers were generally sufficient. Fighters relying on chainmail as their primary defense, which transmits blunt force much more readily, typically needed 4 to 6 layers for adequate cushioning.
The tradeoff was bulk and heat. A thick gambeson under chainmail under plate would add inches of material around the torso and limbs, restricting movement and trapping heat. As plate armor improved through the 14th and 15th centuries and offered better coverage on its own, the trend shifted toward thinner, more tailored arming doublets that prioritized fit and attachment points over raw padding thickness.
Padded Caps Under the Helmet
The head had its own under-layer. A padded arming cap, sometimes called a cale, was a tight-fitting quilted cap worn beneath the helmet. It served three purposes: cushioning the skull from impacts transmitted through the steel, preventing the helmet from shifting during movement or combat, and keeping the metal from rubbing directly against the head. Some versions included a pelerine, a padded extension that draped over the neck and shoulders to distribute the helmet’s weight and protect against pressure points. Without this layer, even a well-fitted helmet would quickly become painful, and a blow to the head would transfer far more force to the skull.
Heat Was the Biggest Problem
All of these layers, topped with steel, created a serious heat management challenge. Research on armored torso coverage shows that full protection can reduce the body’s ability to shed heat by roughly two-thirds compared to an unarmored state. In hot conditions, this limits sustained physical activity to around 60 to 120 minutes before core body temperature rises to dangerous levels. Even in moderate climates, a fully armored knight could reach the edge of heat exhaustion after a couple of hours of exertion.
This is why the choice of under-armor materials mattered so much. Linen against the skin, relatively thin padding in the doublet, and a fitted rather than baggy construction all represented compromises between protection and the body’s need to cool itself. Medieval armorers and tailors clearly understood this balance, even without modern thermodynamic data, because the trend across centuries was consistently toward thinner, more efficient underlayers as plate armor itself became more protective.