One of the
六书.
许慎's definition is only eight characters long, and in his dictionary the example pair
考 and
老 share a radical and are each explained by the other. What he meant has been argued over ever since. A common view, from the Qing scholars
戴震 and
段玉裁, is that it means characters that explain one another, a way of using characters rather than of making them.
