3e SRD:Move Silently Skill
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Move Silently (Dex; Armor Check Penalty)
The character's Move Silently check is opposed by the Listen check of anyone who might hear the character. The character can move up to one-half the character's normal speed at no penalty. At more than one-half and up to the character's full speed, the character suffers a –5 penalty. It's practically impossible (–20 penalty) to move silently while running or charging.