Chart No. 1 · P 20.1
Leading lights (range)
Lights
Two lights on a line: keep them in line and the solid charted line is your safe track.
What it means
Two lights (front lower, rear higher) mark a leading line or range. The solid part of the charted line is the track to follow; it continues dashed where it is only a construction line. A bearing such as '270.5°' is printed along it.
Why it matters
When the two lights appear one above the other you are exactly on the safe track — if the top light slides left, steer left (and vice versa). Ranges are the most precise visual guidance a channel can give.