Rung : Ladder :: Link : ? Complete the analogy.

A ladder is built of rungs set one after another, and a chain is built of links set one after another, so the first word names the repeating piece the whole thing is made of. A rope is made of twisted strands and not of links, a knot is something tied in a rope rather than a piece it is built from, and a bridge may carry a chain but is not made of links.