Armor plate in the 14th Century began as simple knee or elbow cops, then expanded to cover other joints as time went on. By the 15th Century, knights were entirely steel plated. Spaulders are an early form of shoulder armor, extending down over the upper arm.
My spaulders would have been state-of-the-art in the mid 1300s, commonplace by 1400, and obsolete in another 50 years.
This picture of 14th Century spaulders
is a detail of the funeral brass of Sir John de Wautone and wife, dated 1347. You can see the overlapping plates (called "lames") of his spaulders extending from beneath his surcoat.
I bought my spaulders from the Armor Store, which has evolved into SCA World.