The coordinates used in the 1962 formulation were those introduced by Bondi and generalized by Sachs, which focused on null (i.e., light-like) geodesics, called null rays, along which the gravitational waves traveled. The null rays form a null hypersurface, defined by the retarded time u = constant for outgoing waves and advanced time v = constant for incoming waves. The basic idea, which was novel then, was to use the family of outgoing (or incoming) null hypersurfaces to build spacetime coordinates that would describe outgoing (or incoming) gravitational waves. In addition to the retarded (or advanced) time are the space-like distance r and the null-ray direction ( θ θ , φ φ ) to complete the local spacetime coordinates ( u , r , θ θ , φ φ ) . As r is large and approaches infinity, the set of u = constant null hypersurfaces form the future null infinity, where the outgoing gravitational waves "exit". Similar considerations of v = constant null hypersurfaces as r goes to infinity yield the past null infinity, where the incoming gravitational waves "enter". These two null (i.e., light-like) infinities, found using the non-inertial Bondi-Sachs coordinates, are not obvious in the inertial Cartesian coordinates of flat spacetime, where the two time-like infinities and the space-like infinity are obvious. All five infinities are revealed in the asymptotic conformal treatment of infinity by Penrose, where the future (or past) null infinity is denoted by script I + (or script I − − ) and pronounced "scr-EYE plus" (or "scr-EYE minus").