Model of the velocity traces of three points on the dayside near the morning limb (A), substellar point (B), and the evening limb (C). Left panel: schematic of the WASP-121 b system to scale, with orbital phases between quadrature and secondary eclipse. The line of sight is along the vertical axis. Middle panels: model of the cross-correlation traces, assuming that line emission originates from each of the three spots in equal strength, with a line width corresponding to the instrumental resolving power. Planetary rotation causes small differences in the location of each trace. Right panel: co-added velocity–velocity diagram showing the signal due to each of the three points. White dotted lines indicate the true orbital velocity and the velocity at which the substellar point peaks.