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Source: Player's Handbook

Description: You are proficient with medium armor

Prerequisites: Light Armor Proficiency

Required for: Heavy Armor Proficiency

When you wear medium armor with which you are proficient, the armor check penalty for that armor applies only to Balance, Climb, Escape Artist, Hide, Jump, Move Silently, Pick Pocket, and Tumble checks

Special

Fighters, barbarians, paladins, clerics, druids, bards, and any other class listed as having it, automatically have Armor Proficiency (medium) as a bonus feat. They need not select it.

Normal

A character who is wearing armor with which she is not proficient applies its armor check penalty to attack rolls and to all skill checks that involve moving, including Ride.