Boyer–Lindquist coordinates

Summary

Boyer–Lindquist coordinates are the standard coordinates $(t, r, \theta, \phi)$ for the Kerr and Kerr–Newman metrics. They generalize Schwarzschild coordinates to rotating black holes. In them the metric has only one off-diagonal term, $dt\, d\phi$, and its coefficients depend only on $r$ and $\theta$. They are singular on the horizons, as Schwarzschild coordinates are.