Roll: 1d6
Base chance: 1/6
Known By: Thiefers

Thieves have the skill to move silently (i.e. making absolutely no sound whatsoever). Non-thieves cannot sneak unnoticed in silent environments.

Silent environments: Moving silently is required to sneak unnoticed in a completely silent environment. e.g. if a guard is standing quietly listening for intruders, a roll to move silently may be required.
Noisy environments: Thieves can sneak unnoticed without a roll in environments with background noise (i.e. where moving absolutely silently is unnecessary). This includes combat.
Failed rolls: A thief who fails their roll to move silently is still assumed to be moving quietly, thus the normal surprise rules apply).