Jack chains, jackchains
These armour guards were part of the infantry's necessary equipment in the Middle Ages. These soldiers didn't wear plate armour, they only wore vests. These effectively protected the hull, but left hands exposed, which were attacked by swords. Jackchains were such multi-articulated steel straps that were sewn onto the jacket sleeves from shoulder to wrist. Modern studies have shown that these guards prevented up to 80% of injuries from slashing the sword into the shoulder.
