Abstract: A navigational aid assists a person driving a vehicle in maintaining a desired distance from a reference line. It is particularly useful in navigating agricultural vehicles. The navigational aid includes an upright rail mounted adjacent a window in the cab of the vehicle. A marker is carried by the rail and may be positioned on the line of sight from the driver to the reference line. Driving the vehicle with the marker maintained on the line of sight keeps the vehicle at a constant distance from the reference line. The marker may be an element slidably mounted on the rail.