Ō-Yoroi 大鎧

O-Yoroi is a Japanese medieval armour worn between the 10th and 14th Century. Of mostly lamellar construction it covered the trunk of the body and the right flanks (the covering of the right flank was called the waidate): this was to aid the mounted warrior who largely fought on horse back with the bow.  Using a bow meant that carrying a shield was not practicable so shoulder guards called sode were developed to offer protection but maintain freedom of arm movement.

Armour changed in the 14th Century when the samurai fought more commonly on foot and the O-Yoroi gave way to other constructions of armour such as the do maru and the haramaki.

Interesting Armour article here.