An armor check penalty applies to most dexterity-based skills when a character wears armor heavier than leather. This penalty is used instead of the maximum dexterity bonus, and it applies to hide, move silently, parry, pick pocket, set trap, and tumble. The only dexterity-based skills not on this list are open lock and ride, to which neither the maximum dexterity bonus nor the armor check penalty apply. If a character is wearing armor and using a shield, both armor check penalties apply.