Trace of the Perseus arm in the first and second quadrants. Over the longitude range ﹩l=56^\circ \quad {\rm{to}}\quad 180^\circ ﹩ the trace follows the CfA CO survey integrated from ﹩b=-2^\circ \quad {\rm{to}}\quad +2^\circ ﹩. At lower longitudes (within the yellow box) the arm spirals into the far side of the inner Galaxy, where its large distance (﹩\approx 12﹩ kpc) and confusion with near-side emission at the same velocity make the arm difficult to follow. In that region, the higher-resolution CO Galactic Ring Survey (Jackson et al. 2006) is integrated over a half-degree strip of latitude that roughly follows the arm in latitude as judged by visual examination of 21 cm latitude-velocity maps from the VGPS survey (Stil et al. 2006).