Couters are elbow armor. By the mid 14th Century they were jointed, and had a fan or disk of metal on the outside to protect the inside of the elbow.
The couters are tied onto the gambeson with points and laces.
This is a picture of 14th Century couters from the funeral brass of John Foxlay, dated 1378.
I bought my couters from the Armor Store, which has evolved into SCA World.