= Global structure of a charged spherical black hole
{title2=$r_\pm=M\pm\sqrt{M^2-Q^2}$}
For $0<|Q|<M$, a <Reissner-Nordstrom spacetime> has an outer <event horizon> and an inner <Cauchy horizon>. Between them $r$ is timelike; inside the inner horizon $r$ is spacelike and the $r=0$ <curvature singularity> is timelike. The exact analytic extension contains repeated exterior blocks beyond the <Cauchy horizons>. Small perturbations produce severe inner-horizon blueshift and <mass inflation>, so that exact continuation is not a robust prediction for collapse. At $|Q|=M$ the horizons merge into a <degenerate Killing horizon>; for $|Q|>M$ the spherical electrovacuum solution instead has a <naked singularity>.
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