Nikon D6 Reference Manual page 914

D
Virtual Horizon
If you press a control to which [Press] > [Viewfinder virtual horizon] has
been assigned, the camera will display pitch and roll indicators in the
viewfinder based on information from its built-in tilt sensor. These
indicators can be read as follows:
Roll
Camera tilted right Camera level Camera tilted left
862 A Custom Settings: Fine-Tuning Camera Settings