A leading line, called a "range" in the United States, is a line formed by a pair of marks, which are generally man-made, that are used in position fixing

Position fixing

Position fixing is the branch of navigation concerned with the use of a variety of visual and electronic methods to determine the position of a ship, aircraft or person on the surface of the Earth.These techniques include:...

 and navigation

Navigation

Navigation is the process of monitoring and controlling the movement of a craft or vehicle from one place to another. It is also the term of art used for the specialized knowledge used by navigators to perform navigation tasks...

, to indicate a safe passage through a shallow or dangerous channel. If lighted the marks are called "leading lights

Leading lights

Leading lights are a pair of light beacons, used in navigation to indicate a safe passage for vessels entering a shallow or dangerous channel; and may also be used for position fixing. At night, the lights are a form of leading line that can be used for safe navigation...

" in British English and "range lights" in the United States.

Leading lights are often used in the entrances to harbours or estuaries

Estuary

An estuary is a partly enclosed coastal body of water with one or more rivers or streams flowing into it, and with a free connection to the open sea....

where there are hazards close to a deep channel.

Also, in art, Leading Lines are lines that guide your eye through the painting or photograph.