  The longitude separation between adjacent drifting subpulses, $P_2$, is
roughly constant for many pulsars. It was then perplexing when pulsar B0809+74
was found to exhibit substantial variations in this measure, both with
wavelength and with longitude position within the pulse window. We analyze
these variations between 40 and 1400 MHz, and we show that they stem primarily
from the incoherent superposition of the two orthogonal modes of polarization.
