Frequency histograms of the difference between values of systemic velocity from two different sources. The first value, V_sys, is that given in Col. 6 of Table 2 of de Blok et al. (2008). These values were derived from tilted-ring fits to rotation curves of a 19-member subset of THINGS galaxies. The second value, v_fit, was calculated in the present work. In the upper panel, the line centre parameter v_ctr of the fitted profile was used for v_fit; in the lower panel, v_fit is the mean of the low and high velocity values at the 20% height level on the fitted profile.