1. The problem asks to determine the shape of the cross-section formed by a horizontal plane intersecting the left and right faces of a cube. 2. A cube has six square faces, all edges are equal, and all angles are right angles. 3. When a horizontal plane cuts through the cube intersecting the left and right faces, it slices through two parallel square faces. 4. The cross-section formed by slicing a cube with a plane parallel to the top and bottom faces is a square because the intersection is parallel to those faces. 5. Therefore, the cross-section formed is a square. Final answer: The cross-section is a square.