waves in general, and warps in particular, date back over half a century. Much of the discussion parallels efforts to understand spiral structure. In particular, it was recognized early on that in the absence of self-gravity, an initial warp will shear due to differential precession [@kahn1959] in a process akin to the winding problem for kinematic spiral structure. However, @lyndenbell1965 argued that a self-gravitating disc might support true bending modes, which makes warps long-lived. This idea was pursued in detail by @hunter1969 who found that in general, an isolated disc supports a continuum of modes and therefore a generic bending perturbation will disperse. For a disc embedded in a spherically symmetric halo, the only discrete “mode" is the trivial zero-frequency tilt of the disc as a whole. If, on the other hand, the disc is embedded in a flattened halo, then the tilt mode is distorted but remains discrete and is therefore a candidate for explaining long-lived warps. This idea, first proposed by @dekel1983 and @toomre1983, was studied in detail by @sparke1984 and @sparke1988 who treated the disc as a system of concentric rings embedded in the static potential of a flattened halo (see @revaz2001 for a physical justification of 