Body Armour - The First Appearances of True Plate
Prior to the 1300s, body armour was almost exclusively made up of chainmaille and leather defenses. In the thirteenth century, surcoats reinforced with metal plates riveted into the inside of the fabric were beginning to be used, and this idea was refined into a much more functional state with the development of the coat of plates.
The coat of plates was essentially the spirit ancestor of full plate body armour, and was the most commonly used form of body armour up to the 1360s. A coat of plates consisted of a leather or cloth jerkin lined with overlapping plates of iron or steel which were riveted in place. While these were very common, they're rarely seen in illustrations, as while they were worn over the chainmaille hauberk (shirt), they were worn under the surcoat.
The most significant information about the Coat of Plates as we know it came from mass graves found at Wisby. In 1361, it was the sight of a massive battle between the Gotlanders and the Danes. Many died in the battle, and were left at the battlefield in mass graves, and with them, a large part of their equipment. The COPs found there were made of iron plates riveted to T-shaped cloth garment - not like a t-shirt, mind.
In the center of the vertical part of the "T" shape was the hole for a man to put his head through. The arms of the T formed the sides of the coat, and they were lined with plates. When wrapped around to the back, they fastened with a system of buckles and laces made to keep the weight of the plates from dragging it down.
There were variants on the design, but this was the most prevalent one, and likely one of the most effective - which would help explain how common it was.
Wisby was not the only place the COPs showed up, though. Much more well known battles such as Crécy and Poitiers saw a large percentage of the knights fighting wearing these exact sorts of armour.
However, the COPs were not the only form of protection out there. Around about 1350, the art on effigies in particular began to show the gradual evolution of the single-plate breastplate most people think of when they think armour. At this point, however, it was only a plate to cover the upper part of a man's chest, and was attached to rudimentary shoulder protection. In the 1360s, it had evolved to cover down to the diaphragm, with extension plates down to the waist.
It was in the 1370s that the breastplate as we know it came into true being. The lames extending to the waist were gone, replaced with a single piece breastplate that covered down to the hips, with horizontal instead of vertical lames attached at the bottom to go around the waist.
This same time frame aw the emergence of plate arm and leg defenses.
These are quite possibly the most complex parts of armour, given the need for smooth articulation so that the wearer can actually fight wearing it as intended. It was a slow transition from wearing chainmaille chausses on the legs, and the hauberk for the arms, simply because working out mobility in plate was difficult.
Initially, the articulations needed for arms and
legs were handled very simply: leather straps.
The vambrace, or arm guard, was made up of three main parts: the vambrace
proper, which covered from just above the wrist to just below the elbow; the
couter, which was a rounded off piece, often with a fan attached, to guard the
point of the elbow (the fan was intended to provide protection for the inside of
the elbow without impeding movement); and lastly, the rerebrace, which covered
the upper arm. A separate piece called a spaulder was used to cover the point of
the shoulder, thus providing full defense for the arm. The vambrace connected to
the couter with leather straps, and then the couter to the rerebrace much the
same way. The spaulder was generally attached in some way to the body armour,
and held down with a leather strap around the upper arm, overlapping with the
rerebrace.
Much the same was done on the cuisses, or leg guards. The thigh was covered by a large single plate, the cuisse itself, which was attached with articulation to the poleyn, or knee guard, which was usually matched in form to the couter, with the same sort of fan styling. Demigreaves are the smaller plate articulated to the base of the poleyn, and complete the most basic leg armour. Full greaves could be worn, strapped to the shins, by themselves or with the rest of the cuisses. If worn with the rest of the armour, the demigreave overlaps the greave for the sake of full motion.
Eventually the leather straps were replaced with rivet articulation in the later part of the century. Rivet articulation is much more difficult, as it requires lames specially curved to facilitate easy movement placed between each of the major pieces. The best articulation with rivets requires more lames. Fewer will still work - it'll just be stiffer motion. A five-piece knee is, in my experience, far superior to a simpler three-piece one.
It was these sorts of advances in armour construction that was leading up to the far more complex armour of the fifteenth century. However, with advances in armour come advances in weapons to continue to harm the person wearing said armour...and that takes us to the next topic.