  Crescent-shaped asymmetries are common in millimetre observations of
protoplanetary discs and are usually attributed to vortices or dust
overdensities. However, they often appear on a single side of the major axis
and roughly symmetric about the minor axis, suggesting a geometric origin. In
this work, we interpret such asymmetries as emission from the exposed inner
cavity walls of inclined discs and use them to characterise their vertical
extent. Here we focus on the discs around CIDA 9 and RY Tau, first modelling
their observations in visibility space with a simple geometric prescription for
the walls, and then exploring more detailed radiative transfer models.
Accounting for the wall emission yields significantly better residuals than
purely axisymmetric models, and we estimate the dust scale height of these
systems to be 0.4 au at 37 au for CIDA 9 and 0.2 au at 12 au for RY Tau.
Finally, we identify crescent-shaped asymmetries in twelve discs, nine of which
have constraints on their orientation - in all cases, the asymmetry appears on
the far-side of the disc, lending support to the hypothesis that they are due
to their inner rims. Modelling this effect in larger samples of discs will help
to build a statistical view of their vertical structure.
