exist in the stellar *and* dust components of the Milky Way (see also @cox1996 for an earlier discussion of warps in the stellar and gaseous components of external discs).

Warps are but one example of how a galactic disc might deviate from planarity. For example, a disc may also exhibit corrugations or wave-like structures in the direction normal to its midplane. Such structures are most easily observed in the Milky Way where we have access to the full six-dimensional phase space. Consider the Monoceros ring, an overdensity in stars that arcs around the Galactic centre at a Galactocentric radius of about 18 kpc [@newberg2002; @yanny2003; @li2012; @morganson2016]. The Monoceros ring was originally thought to comprise tidal debris from a disrupted dwarf galaxy [@martin2004; @penarrubia2005]. The alternative is that it is a feature intrinsic to the disc. For example, several authors have pointed out that the Monoceros ring arose from warping and flaring of the disc [@momany2004; @momany2006; @hammersley2011] or resulted from the disc interacting with a massive satellite [@kazantzidis2008; @younger2008; @purcell2011; @gomez2016]. More recently, @xu2015 suggested that the Monoceros ring is one of several crests of an oscillatory bending wave observed in the direction of the Galactic anti-centre.

The counterpart 