In order to use some items effectively a character must be proficient with them.
This mostly applies to Armors and Weapons.
Some classes gain proficiency in some armor and weapons as bonus feats when the class is first taken.
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Armor proficiency[]
A character wearing a type of armor with which they are proficient, they apply the armor check penalty for that armor applies to their balance, climb, escape artist, hide, jump, move silently, sleight of hand, and tumble skill checks.
A character who is wearing armor with which they are not proficient also applies its armor check penalty to attack rolls and to all skill checks that involve moving, including ride.
Types[]
and
Weapon proficiency[]
When using a weapon with which a character is not proficient, that character take a –4 penalty on attack rolls.