Some breastplates were made out of one solid sheet of metal, while others, like this example, were comprised of two lames. A lame is a small, thin sheet of metal which is used to create flexible and articulated armour.
Eaves, Ian, Walter J. Karcheski Jr., and Thom Richardson. Armour from Rhodes in the Museum of the Order of St John, Clerkenwell. London: Museum of the Order of St John, 1985.
Karcheski Jr., Walter J., and Thom Richardson. The Medieval Armour from Rhodes. Leeds: Royal Armouries and Higgins Armory Museum, 2000. Pages xi, 63, 141 and 147