Projection of the fundamental and the optical fundamental plane of black hole activity. The LLAGN sample is shown in red stars, and the hard-state XRB sample is shown in blue triangles. The black line is the best-fit line of the complete sample. Luminosities are given in erg s⁻¹ while the masses are in units of solar mass. The relevant references to the X-ray data points are listed in the caption of Table A.3.