Nikon D6 Reference Manual page 192

A
The Viewfinder Display
The viewfinder displays for image area options other than [FX (36×24)]
are shown below. The area outside the current crop is shown in gray.
1.2× (30×20) DX (24×16) 5:4 (30×24)
16:9 (36×20)
1:1 (24×24)
140
Image Recording Options (Image Area, Quality, and Size)