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Source: Player's Handbook

Description: You are proficient with heavy armor

Prerequisites: Light Armor Proficiency, Medium Armor Proficiency

Required for: Efficient Defender

When you wear heavy armor with which you are proficient, the armor check penalty for that armor applies only to Balance, Climb, Escape Artist, Hide, Jump, Move Silently, Sleight of Hand, and Tumble checks.

Special

Fighters, paladins, clerics, and any other class listed as being proficient with it automatically have Armor Proficiency (heavy) 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.