Some vectors transform like contravariant vectors, except that when they are reflected through a mirror, they flip "and" gain a minus sign. A transformation that switches right-handedness to left-handedness and vice versa like a mirror does is said to change the "orientation" of space. A vector which gains a minus sign when the orientation of space changes is called a "pseudovector" or an "axial vector". Ordinary vectors are sometimes called "true vectors" or "polar vectors" to distinguish them from pseudovectors. Pseudovectors occur most frequently as the cross product of two ordinary vectors.