The logical next step from the non-flexible rigidity of a 1-piece breastplate is to split the piece into 2 or 3 pieces arranged vertically. This is how you have the strength and coverage of a 1-piece breastplate but with more flexibility. Fighters need to move! Imagine all of the physical restrictions during a real fight whilst wearing metal plates.
A 3-piece breastplate can be riveted (or strapped) in the middle, thus allowing some side-to-side movement, or it can be attached on the outsides and crafted to allow some up-down movement. Of course fighters need both, so some hybrids are also possible.
More soon…