 Kruskal–Szekeres coordinates

Kruskal–Szekeres coordinates extend the Schwarzschild solution smoothly through its future and past horizons. Constant Schwarzschild radius curves are hyperbolae, while the horizons are null lines through the bifurcation sphere.

Kruskal–Szekeres coordinates are a specific set of coordinates used in the context of general relativity, particularly to describe the Schwarzschild solution, which describes the spacetime surrounding a spherically symmetric, non-rotating mass such as a black hole. These coordinates are particularly useful because they allow for a smooth and complete description of the Schwarzschild black hole, including regions that might be singular or undefined in standard Schwarzschild coordinates.