Proficient

Being proficient with a weapon, shield or armor means you can use it without incurring the non-proficiency penalties. Proficiencies are obtained through feats, either given at character creation or during advancement.

Proficiency feats

 * Simple Weapon Proficiency
 * Martial Weapon Proficiency (per weapon)
 * Exotic Weapon Proficiency (per weapon, requires base attack bonus of +1 or higher)
 * Light Armor Proficiency (all)
 * Medium Armor Proficiency (all, requires Light Armor Proficiency)
 * Heavy Armor Proficiency (all, requires Medium Armor Proficiency)
 * General Shield Proficiency (all except tower)
 * Tower Shield Proficiency (requires General Shield Proficiency)

Non-proficiency penalties

 * Gives -4 to attack rolls if not proficient with weapon.
 * Applies item's Armor Check Penalty to attack rolls if not proficient with armor or shield. (ACPs apply to skills regardless of proficiency.)
 * Note that an armor or shield with an ACP of 0 can be used without penalty by all characters whether they're proficient with it or not.
 * Mithral/darkwood heavy shield has 0 ACP.
 * Mithral light shield has 0 ACP and 0% ASF.