Poleyns are knee armor. By the mid-14th Century they were jointed, and sometimes had a fan or disk of metal on the outside to protect the back of the knee.
My poleyns are tied onto the gamboised cuisses with points and laces.
This picture of 14th Century poleyns is a detail from the funeral brass of Sir John d'Abernoun the Younger, who died in 1327. It's not entirely clear whether these poleyns are jointed, or whether they simply have a flared edge. To my eye, they look jointed.
I bought my poleyns from the Armor Store, which has evolved into SCA World.