Differences between the low-frequency central frequencies observed in 3960 days of GONG data (1 ⩽ l ⩽ 35) using the m-averaged spectrum technique and the associated theoretical values, taken from Christensen-Dalsgaard's model S (Christensen-Dalsgaard et al. 1996), as a function of the angular degree (left panel), of the frequency (middle panel), and of the inner turning point (right panel). The uncertainties multiplied by a factor of 20 are also presented. Ridges of same radial orders are connected, and for guidance some of the corresponding radial orders are indicated in the middle panel.