  The standard models of sequence evolution on a tree determine probabilities
for every character or site pattern. A flattening is an arrangement of these
probabilities into a matrix, with rows corresponding to all possible site
patterns for one set $A$ of taxa and columns corresponding to all site patterns
for another set $B$ of taxa. Flattenings have been used to prove difficult
results relating to phylogenetic invariants and consistency and also form the
basis of several methods of phylogenetic inference. We prove that the rank of
the flattening equals $r^{\ell_T(A|B)}$, where $r$ is the number of states and
$\ell_T(A|B)$ is the parsimony length of the binary character separating $A$
and $B$. This result corrects an earlier published formula and opens up new
applications for old parsimony theorems.
  Since completing this work, we have learnt that an equivalent result has been
proved much earlier by Casanellas and Fern\'andez-S\'anchez, using a different
proof strategy.
