A jack or jack of plate is a type of armour comprising small iron plates sewn between layers of felt and canvas.

The jack is similar to the brigandine

Brigandine

A brigandine is a form of body armour from the Middle Ages. It is a cloth garment, generally canvas or leather, lined with small oblong steel plates riveted to the fabric....

, the main difference is in the method of construction: a brigandine is riveted whereas a jack is sewn.

Jacks of plate were created by stitching small overlapping squares of iron in between two canvases. The garments weighed about 17 pounds, which made them much more pleasing to wear than solid breastplates. They allowed soldiers with weapons to rest the butts of the weapons firmly against their shoulders, which wasn't feasible with smooth surface plate armours. Primarily, they were used in England during the medieval times. The present day equivalent of a jack of plate is a bullet-proof vest.