  Chromium trihalides, CrX$_3$ (with X = Cl, Br, I), are a family of layered
magnetic materials that can be easily exfoliated to provide ferromagnetic
monolayers. When two layers are stacked together to form a bilayer the
interlayer exchange coupling can be either ferromagnetic or antiferromagnetic
depending on the stacking sequence. Here we combine crystallographic arguments
based on the close-packing condition with first-principles simulations to
enumerate all possible stacking patterns in CrX$_3$ bilayers that preserve the
spatial periodicity of each layer. We recover all configurations observed in
bulk crystals and disclose stacking sequences with no bulk counterpart where
the two layers have opposite chirality. Stacking sequences are ranked according
to their relative stability and a preferential interlayer magnetic ordering is
assigned to each of them. Simulations provide a consistent picture to frame all
current experimental observations on bulk and exfoliated CrX$_3$ crystals, with
interesting implications for future measurements, including synthetic bilayers
with non-standard stacking patterns.
