  The question of how ferroelectric polarization is coupled to magnetism in
magnetoelectric multiferroics, in which both types of order are simultaneously
present, is of considerable scientific and practical interest. A recent Letter
\cite{fennie} presents an analysis of the important ``ABO$_3$'' class of
perovskite multiferroics. This Letter argues that antiferromagnetic
multiferroics with magnetic ions on the B site, such as the well-studied
room-temperature multiferroic bismuth ferrite (A=Bi, B=Fe), cannot show linear
magnetoelectric coupling of the form ${\bf P} \cdot ({\bf L} \times {\bf M})$.
Here ${\bf P}$ is polarization and ${\bf L}$ and ${\bf M}$ are
antiferromagnetic and ferromagnetic moments. The conclusion of Ref.
\onlinecite{fennie} is that only materials with magnetic A-site have this
coupling. This Comment presents a compact analysis of magnetoelectric coupling
in the ABO$_3$ multiferroics. We show that the argument of Ref.
\onlinecite{fennie} does forbid $E_{PLM}$ if the final low-symmetry phase
contains only one distortion that, like ${\bf P}$, breaks all inversion
symmetries. In reality, there are multiple distortions in this symmetry class,
and cross-terms generate $E_{PLM}$. Our analysis gives simple conclusions about
existence and optimization of magnetoelectric coupling in ABO$_3$ materials.
