See also: Feats
You are proficient with Medium Armor, and can wear it without impediment.
Prerequisite: Armor Proficiency (Light) Feat
Effect: When you wear Medium Armor, you take no Armor Check Penalty on attack rolls or Skill Checks. Additionally, you benefit from all of the armor's special Equipment bonuses (If any).
Normal: A character who wears Medium Armor with which they are not proficient takes a -5 Armor Check Penalty on attack rolls as well as Skill Checks made using the following Skills: Acrobatics, Climb, Endurance, Initiative, Jump, Stealth, and Swim. Additionally, the character gains none of the armor's special Equipment bonuses.