Example for the definition of δ _{i, j −} and δ _{i, j +} (see the text). Four radial orders of l = 1 modes from three adjacent models in a high-resolution grid of solar models are shown. The triangles represent the central model. The frequencies for the adjacent models along the evolutionary track sequence are depicted as squares and white circles. Black circles (and error bars) indicate observed frequencies published in Broomhall et al. (2009). δ _{i, j −} and δ _{i, j +} for a single mode are represented using arrows. The insert shows an unzoomed version of the l = 1 and l = 3 ridge.