Diagonal (Kosumi) in Go — A Solid Connection | PlayGo.gg

When a stone is played diagonally to another stone. Think of this as how bishops move in chess.

What is a kosumi?

A kosumi is a stone placed diagonally next to one of your existing stones, with the two intervening points empty. The two stones touch only at the corner, yet the shape behaves as a single unit — the two empty points form a miai pair, so whichever one the opponent plays, you take the other and connect underneath.

The word literally means "small corner" in Japanese, which is exactly what the shape resembles. On an empty board it is uncuttable; the only situations where a kosumi cut succeeds involve outside forces (a ladder breaker, a stone in atari, a complex squeeze).

When to use a kosumi?

The kosumi is the slowest of the standard "stretching" moves — one diagonal step is very little board coverage — but in return you get a shape that cannot be cut, is hard to attack, and pushes in two directions on the next move.

Reach for a kosumi when strength matters more than speed: defending a weak group, securing a base in the corner, or anchoring a group that must live. Avoid it in the opening where both sides are racing to claim large frameworks — a single diagonal step is usually too slow there.

The mental ladder of stretching moves: kosumi (one diagonal step, uncuttable, slow) → one-space jump (faster, almost never cut) → knight's move (faster still, genuinely cuttable in a fight). The further apart the two stones, the more speed and risk.

Example of a kosumi in a game

How do we connect the C2 stone back to the center stones via a diagonal move?

The kosumi is Go's safest stretching move. Play it when the group must not be cut. Skip it when speed matters more than safety — and trust the geometry: an empty-board diagonal is exactly as strong as a solid line of stones.