Shear as the common ground between the magneto-rotational and magneto-elliptical instabilities. The distance between two points in uniform rotation does not increase if the streamlines are circular, i.e., the rotation is rigid (upper figure). However, in elliptic streamlines the distance between the two points does increase even if the rotation is uniform (lower figure). A magnetic field connecting the two points will resist this shear, leading to instability depending on the field strength.