The breastplate is the shaped steel plate over the chest, and the piece a plate harness is built around.
The breastplate is the shaped steel plate covering the chest, and it is the piece around which a plate harness is assembled. Everything else — the gorget above, the fauld below, the arm defences at the sides — attaches to it or against it.
Its arrival marks the real transition to plate armour. As long as the torso was protected by mail or by a coat of plates, a harness was an accumulation of parts; once a single shaped plate covers the chest, the harness becomes a designed object.
It is also the piece that lasted. Long after helmets, limbs and gauntlets had been abandoned, cavalry were still riding out in a breastplate and backplate, and cuirassiers wore them into the twentieth century.
It is raised from a single plate of steel by hammering over stakes, and the difficulty is the compound curvature: the surface has to swell over the chest and tuck under the ribs without cracking or thinning.
That shaping is the armour. A curved surface presents a glancing face to almost every angle of attack, so a point that would bite into a flat plate skids off instead, and a breastplate of two to three millimetres does work that a much heavier flat plate could not.
Below it hangs a fauld of horizontal lames covering the hips, and on the right side sits the lance rest — a bracket that takes the shock of a couched lance into the wearer's body rather than into his arm. From the mid-fifteenth century many are made in two pieces, a lower plackart overlapping an upper plate so the torso can bend.
Against every hand weapon of the period a good breastplate is effectively proof. Cuts glance, thrusts skid, and the fifteenth-century fight books do not attempt to defeat it at all — they attack the gaps instead.
It is what makes the dismounted man-at-arms of the fifteenth century so formidable. At the Battle of Agincourt in 1415 the French advanced on foot in full harness, and what stopped them was mud, exhaustion and the press rather than any weapon reaching their chests.
Mounted, it carries the lance rest and therefore the entire mechanics of the charge. The breastplate is what allows a rider to deliver the momentum of horse and man through a lance point without being driven out of the saddle.
Its strengths are protection and load-bearing. It stops what the period could throw at it, and it distributes the weight of the harness across the torso rather than hanging it from the shoulders.
Its limitations are rigidity and heat. A one-piece breastplate does not bend, so a man in an early form cannot stoop easily — which is exactly what the two-piece plackart construction was invented to fix.
Firearms eventually changed the calculation rather than defeating it outright. Breastplates were made thicker and "proofed" by being shot at, and the dent left by the proof shot became a maker's guarantee — but the added weight is what finally drove the rest of the harness away.
The coat of plates and the brigandine cover the torso with many small plates through the fourteenth century, and the single breastplate emerges from them around 1400 as armourers gained the skill to raise one large piece.
The first half of the fifteenth century produces the two great regional forms: the German Kastenbrust, boxy and angular with a flared skirt, and the Italian globose breastplate, smooth and rounded.
From mid-century the two-piece construction spreads, and after 1500 the form changes with fashion — peascod shapes, heavier proof plates — before contracting to the cavalry cuirass that outlives everything else.
The Italian globose form deflects by curvature alone and is characteristic of the Milanese workshops that armed much of Europe. It is smooth, unfluted and shaped to the body.
The German Kastenbrust is its opposite: an angular box-like chest with a sharply flared lower edge, often finished with radiating fluting, and it is one of the most recognisable armour forms of the mid-fifteenth century.
German practice then moves on to the fluted Gothic style, where the fluting stiffens a thinner plate, and the two traditions converge only in the sixteenth century.