Omittable Planes

In analogy to omittable lines in the plane, we initiate the study of omittable planes in 3-space. Given a collection of n planes in real projective 3-space, a plane Π is said to be omittable if Π is free of ordinary lines of intersection – in other words, if all the lines of intersection of Π with other planes from the collection come at the intersection of three or more planes. We provide two infinite families of planes yielding omittable planes in either a pencil or near-pencil, together with examples having between three and seven omittable planes, examples that we call “sporadic,” which do not fit into either of the two infinite families.