Poleyns are the leg equivalent of the couter are are plate defences for the knee. They are sometimes referred to in the SCA, as knee cops.
Cup shaped defences appear in the middle of the 13th century and were laced or strapped to chausses. By the 3rd quarter of the 14th century poleyns developed a heart shaped wing. The poleyns became riveted to the cuisse with articulated lames later that century.
In the early 15th century a small fringe of mail might be attached to the poleyn (although not when the lames attaching the poleyn to the greave were pointed).