Properly embedded simplices in a convex divisible domain Ω ⊂ RP d behave somewhat like flats in Riemannian manifolds [Sch90], so we call them flats. We show that the set of codimension-1 flats has image which is a finite collection of disjoint virtual (d − 1)-tori in the compact quotient manifold. If this collection of virtual tori is non-empty, then the components of its complement are cusped convex projective manifolds with type d cusps.