The coordinate vector field ∂ ∂ u can be spacelike, null, or timelike at a given event in the spacetime, depending upon the sign of H ( u , x , y ) at that event. The coordinate vector fields ∂ ∂ x , ∂ ∂ y are both spacelike vector fields. Each surface u = u 0 , v = v 0 can be thought of as a wavefront.