This quantity can be used to examine the 'boundaries' of an accretion disc: when κ κ 2 becomes negative, then small perturbations to the (assumed circular) orbit of a fluid parcel will become unstable, and the disc will develop an 'edge' at that point. For example, around a Schwarzschild black hole, the innermost stable circular orbit (ISCO) occurs at three times the event horizon, at 6 G M / c 2 .