Visual definition of the velocity amplitude (v_amp) and velocity shear (v_shear) using as an example the systemic component of SDSS 0741+3020. Top: red dotted horizontal lines identify the maximum (v_max) and minimum (v_min) velocity values used in the v_amp definition. Bottom: red dotted horizontal lines identify in this case and , as defined in Sect. 4.2, used in the v_shear definition. The blue and green arrows mark v_amp and v_shear in both diagrams.