The breastplate is the earliest piece of metal body armor. It covered the front of the body between the neck and waist, protecting the chest area. It was often attached to a backplate by hinges or leather straps over the shoulders and under the arms. Breastplates are dateable because they reflect male civilian clothing. Early breastplates were very rounded with a short bottom flange, mirroring the current fashion of the cloth doublet or jacket. Later, following changes in the doublet which swelled out and under at the stomach, the breastplate developed a pronounced central ridge, running from the neck to the waist. Its lines became elongated, forming a very pronounced "V" to the front, which, on the doublet, accommodated the short puffy breeches that were then in vogue.

The steel breastplate from James Fort, which is short and rounded, is of the early variety and probably dates to the beginning of the 16th century. The underarm areas are not rounded to prevent the edges from digging into the wearer's undergarments, as they would be on early 17th-century breastplates. The James Fort breastplate was presumably furnished with separate plate elements at the underarms known as gussets, which allow for freer arm movement.

Breastplate, steel, early 16th century. Context: Pit 1, ca. 1607-1610.