A diagram used to plot trajectories in spacetime near a black hole. The vertical and horizontal coordinates, called Kruskal coordinates, are two complicated functions of time and distance from the black hole. Lines of constant time radiate from the origin of the diagram, with steeper slopes corresponding to later times. Photons always travel along diagonal lines at ±45° to the vertical. The trajectory of an object falling into the black hole is shown as a curving line moving upwards on the diagram at less than 45° to the vertical. The diagram is named after the American physicist Martin David Kruskal (1925–2006).