  Two-body charmless nonleptonic decays of B_u and B_d mesons are studied
within the framework of generalized factorization in which the effective Wilson
coefficients $c^{eff}_i$ are renormalization-scale and -scheme independent
while factorization is applied to the tree-level hadronic matrix elements.
Contrary to previous studies, our $c_i^{eff}$ do not suffer from gauge and
infrared problems. Nonfactorizable effects are parametrized in terms of N_c(LL)
and N_c(LR), the effective numbers of colors arising from (V-A)(V-A) and
(V-A)(V+A) four-quark operators, respectively. Tree and penguin transitions are
classified into six different classes. The data of $B^-\to\rho^0\pi^-$ and
$B^-\to\phi K^-$ clearly indicate that $N_c(LR)\neq N_c(LL)$: The first
measurement of the b -> u mode $B^-\to\rho^0\pi^-$ and the experimental
information on the tree-dominated mode $B^-\to\omega\pi^-$ all imply that
Nc(LL) is less than 3, whereas the CLEO measurement of $B^-\to\phi K^-$ shows
Nc(LR)>3. For given input parameters, the prediction of ${\cal B}(B\to\eta' K)$
is largely improved by setting Nc(LL)~2 and Nc(LR)>Nc(LL); in particular, the
charm content of the eta' contributes in the right direction. The decay rate of
$B\to\phi K^*$ is very sensitive to the form-factor ratio A_2/A_1; the absence
of $B\to\phi K$ events does not necessarily invalidate the factorization
approach. If the branching ratio of $B^-\to\omega K^-$ is experimentally found
to be significantly larger than that of $B^-\to\rho^0 K^-$, we argue that
inelastic final-state rescattering may account for the disparity between
$\omega K^-$ and $\rho^0 K^-$.
