Perhaps this example may clarify; Join two tetrahedrons together you get an octahedron. Now the 2D plane dividing the octahedron belongs to both tetrahedrons. It is the bottom plane of each tetrahedron at the same time, but one tetrahedron is pointing up, the other tetrahedron is pointing down. But they share the same bottom plane, one side facing up, the other side facing down. It is a shared 2D aspect (plane) of both tetrahedrons. The same as 24:00:00, as the end of one 24 hr set (day) equals 00:00:00, as the beginning of the next 24 hr set (day).