The perifocal frame is the two-dimensional frame of reference for an orbit. It is centered at the focus of the orbit, i.e. the planet about which the orbit is centered. Its plane is given the coordinates p and q, where the p axis is directed toward the periapsis of the elliptical orbit (in the direction of the eccentricity vector). The third dimension of this orbit, w, is composed of the normalized angular momentum vector, which is the radius vector crossed with the velocity vector.