A *pencil of planes* in
is a family of planes having a
common line of intersection. The cross ratio of four planes of a pencil is the same as the cross ratio of the lines *l*_{i} of
intersection of the planes with fifth, transversal plane (see
fig. 3.3).

Once again, different transversal planes give the same cross ratio as the figures they give are projectively equivalent. The Möbius formula also extends to this case: let

where stands for a determinant of 4-component column vectors.