A ladder rung is an imaginary line at right angles to the wind. Boats on the same rung have made equal progress upwind, even when they are far apart.
Why it matters: rungs help you see who is ahead, recognise gains and losses, and understand why a shift helps one side. They turn with the wind; the direction of the mark alone does not tell you which tack is lifted.
Video and original lesson: SailZing 路 Ladder Rungs 路 Watch on YouTube