Longitude–velocity diagram of CO emission from the CfA 1.2-m survey (Dame et al. 2001). The shaded region denotes the trajectory of the Sagittarius arm from Burton & Shane (1970). The width of the shaded region corresponds to a ±5 km s^-1 velocity dispersion. Dots indicate the HMSFRs assigned to the arm based on the longitude–velocity diagram; squares are HMSFRs assigned to the arm based on distances. Error bars reflect the V_LSR uncertainties of the HMSFRs listed in Table 2.