Bad knight

A bad knight is not a defective piece and does not have permanently reduced value. It is a knight that, in the current position and foreseeable future, lacks useful squares, targets, or an efficient route to the important area of the board. It may sit on the rim, be chased repeatedly by pawns, or remain tied to a defensive duty.

Visible mobility is only part of the evaluation. Ask where the knight can reach in two or three jumps, whether those squares are stable, and what it will do there. An outpost is a protected square in enemy territory from which opposing pawns cannot easily drive the piece away. Without such a square, even in a closed position, where pawns block lines, the knight may remain ineffective.

The knight can be improved by planning a route, supporting a new square with a pawn, making a pawn break, meaning an advance intended to force exchanges and open lines, freeing it from defense, or exchanging it for a more useful enemy piece. This must fit a plan, because a slow maneuver may allow activity elsewhere.

In a bishop versus knight comparison, the rule that knights prefer closed positions is not enough. The knight's actual squares and the bishop's ability to act while it reorganizes are what matter.