The Scent Border is the boundary marking the border of the Mouse Territories. Comprising a long stick fence supplemented in places by a row of low rocks, the Scent Border is not of itself a defensible perimeter. Rather, a repellent mixture which is poured along the boundary itself repels larger animals away from mousekind.
The scientists of Sprucetuck concoct the repellent mixture, and the Mouse Guard repour it at least twice a year. This task is one of the most vital carried out by the Guard, so its execution is placed into the paws of only the most trusted patrols.
Many mice do not even know of the Scent Border's existence, and only a few living mice have ever gone beyond its boundaries.