A Plate Mail Body (aka. a platebody) is a piece of armour worn on the torso by male and female characters. They offer significantly more protection than a chain mail body does at a given tier, but have exactly the same stats as a respective plate mail top. However, players cannot equip gloves or gauntlets while wearing one, which can be done while wearing a chain mail body.
They can be made by Smithing 5 metal bars on an anvil while carrying a hammer in the player's inventory. They can also be exchanged for plate mail tops (worn by female characters) and vice-versa by using it on Thrander, located south of the east bank in Varrock. The exchange is done for free.
Types from bronze to mithril can be purchased from Horvik's Armour Shop in Varrock. Adamantite is sold at Valaine's Shop of Champions within Champions' Guild, while rune can be equipped and bought from Oziach near Edgeville after completing the Dragon Slayer quest.