  We perform an isospin analysis of $B$ decays to two pseudoscalars. The
analysis extracts appropriate CKM and short distance loop factors to allow for
comparison of non-perturbative QCD effects in the reduced matrix elements of
the amplitudes. In decays where penguin diagrams compete with tree-level
diagrams we find that the reduced matrix elements of the penguin diagrams,
which are singlets or doublets under isospin, are significantly enhanced
compared with the triplet and fourplet contributions of the weak Hamiltonian.
This similarity to the $\Delta I =\frac{1}{2}$ rule in $K \to \pi \pi$ decays
suggests that, more generally, processes mediated by Hamiltonians in
lower-dimensional isospin representations see enhancement over
higher-dimensional ones in QCD.
