Triangulation is an endgame technique in which a king takes a three-square detour — tracing a triangle — to transfer the move to the opponent, placing them in zugzwang (a situation where any move worsens their position).
The technique works because of an asymmetry in king mobility. The triangulating king can reach a target square in either two or three moves depending on which path it takes, while the opposing king — confined to a narrower area — has no equivalent flexibility and cannot mirror the detour. This forces the opponent to yield ground or make a concession. Triangulation is essentially a form of indirect opposition and is often the decisive winning idea in king-and-pawn endings.
In practice, ask yourself whether your king can reach a key square in either an even or odd number of moves, and whether your opponent’s king cannot do the same. If that asymmetry exists, executing a triangulation lets you deliberately "waste a tempo" to flip the opposition in your favor.