  Hadron resonances affect nonexotic $D^o$ decays but not $B$ decays which are
far from the resonance region. We obtain new information from {\bf exclusive}
decays and show that interference between colour favoured and colour suppressed
diagrams is {\bf constructive} in $B$ and (some) $D$ decays in contrast to the
inclusive decays where a net Pauli destructive interference is claimed. We
suggest that a systematic study of $B$ decay final states containing excited
mesons such as $a_1 D$ or $\pi D^{**}$ show opposite behaviour for $B^o$ and
$B^+$ relative to the ground state channels. We give inequalities among final
state branching ratios for several $B$ and $D$ channels. Decays into $\pi
K^{(*)}$ and $\rho K^{(*)}$ appear to show different behaviours.
