  We discuss a geometrical interpretation of the Z-invariant Ising model in
terms of isoradial embeddings of planar lattices. The Z-invariant Ising model
can be defined on an arbitrary planar lattice if and only if certain paths on
the lattice edges do not intersect each other more than once or self-intersect.
This topological constraint is equivalent to the existence of isoradial
embeddings of the lattice. Such embeddings are characterized by angles which
can be related to the model coupling constants in the spirit of Baxter's
geometrical solution. The Ising model on isoradial embeddings studied recently
by several authors in the context of discrete holomorphy corresponds to the
critical point of this particular Z-invariant Ising model.
