An Excerpt From Jamestown Rediscovery III by William M. Kelso, Nicholas M. Luccketti, and Beverly A. Straube:
Breastplates, protecting the chest area, were integral parts of armor worn from the 15th through the 17th centuries. Because they stylistically reflect male civilian clothing, they are easily datable. Early breastplates, like the one found in James Fort, were very rounded with a short bottom flange, mirroring the current fashion of the cloth doublet or jacket. Later, following changes in the doublet, the breastplate developed a pronounced central ridge, running from the neck to the waist, and the lines become elongated, forming a very pronounced "V" to the front. The ridge not only provides a glancing surface to the blow of a sword or pike, but is an interpretation in steel of the effect made by the row of tiny round brass buttons running down the front of the doublet. The high V-shaped bottom of the breastplate copied the cut of the doublet which accommodated the short puffy breeches that were then in vogue.
As already mentioned, the excavated breastplate is of the very rounded 15th-century type. The neck and underarm edges that would normally be rolled for the wearer's comfort show signs of being cut down. This has resulted in the breastplate being very narrow through the chest area, perhaps providing a better fit for the Jamestown soldier lucky enough to wear it.