Strategy guide · Cham-gonu 참고누

The running mill: how Cham-gonu is really won

Cham-gonu is the deepest game in Korea's Gonu family — two players, 12 pieces each, a 24-point board of three nested squares. You can lead the whole first half and still lose, and almost always for the same reason: the other player built a running mill. This guide walks through a real game where exactly that happens, then shows how to build one yourself and how to stop one.

157 of 160games were won by the side that built a running mill, whenever only one side had one. (A running mill appeared in 164 of 400 games between our strongest AIs.)

The board in 30 seconds

The 24 points sit on three nested squares. Lines join the middle of each side across all three squares, and — unlike the Western Morris games — the corners are joined by diagonals too. That makes 20 lines where three of your pieces form a mill. Not every point is equal:

Point	Neighbours	Lines through it
Middle square, corner	4	3
Middle square, side centre	4	2
Outer / inner square, corner	3	3
Outer / inner square, side centre	3	2

The four middle-square corners are the best points on the board: the most room to move and the most mills to join. The opening move itself is restricted — it has to go on one of the four outer corners.

Watch it happen: a real game

Red moves first. Keep an eye on Blue's bottom side — it looks harmless for most of the first half.

The opening move: Red places on the bottom-right outer corner; the four outer corners are circled. — Move 1The first move goes on an outer cornerOnly the four outer corners are legal for the very first piece. Red starts bottom-right.

Red's three pieces on the right side glow as a mill; the captured point on the left is circled. — Moves 9–10Three in a row is a mill — and a captureRed lines up three on the right side and removes a blue piece. The emptied point is marked × and stays off-limits until the first half ends.

A diagonal mill from the top-left outer corner to the inner square glows. — Moves 22–23Diagonals count tooTop-left corner straight down the diagonal — Red's third mill. These corner diagonals are what set Cham-gonu apart.

Blue's pieces on the middle and inner bottom sides are circled, with a two-way arrow along the bridge between them. — Move 26Blue's frame is finishedThree on the middle bottom side, both ends of the inner bottom side, and the bridge that joins them. The gap at the end of the bridge is a point Blue emptied earlier by capturing.

The board at the end of the placing half, Red with 11 pieces and Blue with 8. — Halftime11 against 8 — Red is clearly aheadRed built four mills while placing. From here pieces slide one point at a time instead of being placed.

Both of Blue's bottom mills glow, with an arrow showing one piece sliding back and forth between them. — Moves 32–36The running millBlue slides one piece down the bridge — inner mill, capture — and back up — middle mill, capture. Every single move closes a mill.

The red piece that stepped next to the gap is circled; Blue captured it straight away. — Moves 37–39Trying to plug the gapRed slides next to the gap to block it, and Blue simply captures that piece first. The inner gap is surrounded by blue pieces, so Red can't reach it at all.

The final position: Red is down to two pieces and Blue wins. — Move 57Nine captures in a rowRed drops to two pieces and loses — from 11 against 8, without Blue ever needing a second idea.

How to build a running mill

Two parallel sides and one bridge. Take two sides that are joined by a straight spoke — for example the middle and inner bottom sides. Hold both corners of each side, and keep one piece on the bridge.
Make the gap unreachable. The point your shuttle leaves must not border an enemy piece, or they step in and jam it. A side centre on the outer or inner square is ideal: its only neighbours are your two corners and the bridge.
Build it while placing. Once the second half starts, nothing new can be placed. Spend first-half moves on the frame even if it means letting your opponent win a few mills — Blue did exactly that.
Captures can open the gap for you. A point you empty by capturing during placement can't be filled again until moving starts — a ready-made hole for your bridge.

How to stop one

Take a frame point early. A running mill needs five points. Occupy any one of them during placement and the whole thing collapses.
Block the gap — before the mill closes. Stepping next to the gap only works if your piece survives; as the game above shows, the running mill captures the blocker first.
Counter-attack the open side. Whenever the shuttle leaves a mill, the two pieces it left behind are no longer protected. Close a mill of your own and capture one of them.
Count mobility. If they have no legal move on their turn, you win outright — a crowded board makes blockades common.

Try it yourself

The Master AI builds and defends running mills. See if you can get yours working first.

Play Cham-gonu vs AI Online battle Full rules