Like Ron said, 9" on either side of unit, or more specifically C clamps 18" apart unless its a double clamp on a mover. Our marks are based off of the Centerline. So Center is our 0 and then on either side we've got 1'6, 3' etc going off towards the wings. We can accommodate whatever the designers want, we just found that even if a plot isn't based around 18" our marks are close enough to quickly figure out where their lights will land. It's certainly sped up lighting calls since we've done it.Yes, but does the 18" split center? or center unit? What do you do if a designer wants the other?