When Roy Kerr developed the Kerr metric for spinning black holes in 1963, he ended up with a four-parameter solution, one of which was the mass and another the angular momentum of the central body. One of the two other parameters was the NUT-parameter, which he threw out of his solution because he found it to be nonphysical since it caused the metric to be not asymptotically flat, while other sources interpret it either as a gravomagnetic monopole parameter of the central mass, or a twisting property of the surrounding spacetime.