Mithral Armor

Mithral is a light, flexible metal, reducing the type of armor by one. Light becomes "clothing", Medium becomes Light, Heavy becomes Medium.

A mithral chain shirt or breastplate can be worn as underclothes.

If the armor normally imposes disadvantage on Dexterity (Stealth) checks or has a Strength requirement, the mithral version of the armor doesn't. 

Adds 2 to the maximum AC modifier from Dexterity for Medium armor for a maximum of 4 AC from your Dexterity modifier.