Geometry of the emission models. The angle between the observer’s line-of-sight and the BH spin axis is defined as the inclination angle (i). The orientation of the spin axis on the plane of the sky is the position angle (PA). A completely face-on accretion disk occurs at i = 0°, an edge-on disk is seen at i = 90°. The spin axis points due north when PA = 0°, and due east when PA = 90°, orientation changes east of north (i.e., counterclockwise).