3e SRD:Armor Proficiency (light)
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Armor Proficiency (light) [General]
Benefit: When a character wears a type of armor with which the character is proficient, the armor check penalty applies only to Balance, Climb, Escape Artist, Hide, Jump, Move Silently, Pick Pocket, and Tumble checks.
Normal: A character who is wearing armor with which she is not proficient suffers its armor check penalty on attack rolls and on all skill checks that involve moving, including Ride.