Leading lights

The beacons consist of two lights that are separated in distance and elevation, so that when they are aligned, with one above the other, they provide a bearing.

At night when viewed from a ship, the two lights only become aligned vertically when a vessel is positioned on the correct bearing.

[4] Leading lights were used in Great Britain as early as 1763 to mark the Port of Liverpool.

[5] The first set of range lights in the United States were privately established by subscription at Newburyport Harbor in Massachusetts in 1788.

[6] Leading lights are sometimes designed to be movable, allowing their position to be shifted in the event of a change in the safe channel; these include one at Hilton Head, South Carolina, the original Chatham Light, and the Nantucket Beacon, predecessor to the Nantucket Harbor Range shown above.

Nantucket Harbor Range Lights [ 1 ] The dayboards are type KRW, see below.
A pair of leading lights in Bremerhaven , Germany, with the rear light in a proper lighthouse and the front light on a smaller tower. No standardised markings are used here.