  We define the collinear factorization scheme, which absorbs only the
collinear physics into the parton distribution functions. In order to isolate
the collinear physics, we introduce a procedure to combine real and virtual
corrections, canceling infrared singularities prior to integration. In the
collinear scheme, the factorization scale $\mu$ has a simple physical
interpretation as a collinear cutoff. We present a method for choosing the
factorization scale and apply it to the Drell-Yan process; we find $\mu \approx
Q/2$, where $Q$ is the vector-boson invariant mass. We show that, for a wide
variety of collision energies and $Q$, the radiative corrections are small in
the collinear scheme for this choice of factorization scale.
