  The "universal" instability has recently been revived by Landreman, Antonsen
and Dorland [1], who showed that it indeed exists in plasma geometries with
straight (but sheared) magnetic field lines. Here it is demonstrated
analytically that this instability can be present in more general sheared and
toroidal geometries. In a torus, the universal instability is shown to be
closely related to the trapped-electron mode, although the trapped-electron
drive is usually dominant. However, this drive can be weakened or eliminated,
as in the case in stellarators with the maximum-$J$ property, leaving the
parallel Landau resonance to drive a residual mode, which is identified as the
universal instability.
