Diagram showing the four diagnostics used to infer (variability) properties of double-peaked emission lines. The peak-to-peak velocity corresponds to the velocity difference between the central velocities of the two peaks (orange). The heights of the peaks are divided to obtain the V/R ratio (green). The difference between the middle of the central peaks and the rest wavelength gives a central wavelength shift or radial velocity shift (purple). The equivalent width of this line is determined between −250 and −150 km s⁻¹ and between 150 and 250 km s⁻¹ (red). The (V/R)_wing ratio is the ratio of these values.