Learn Nine Men's Morris

Each player has nine men. The board is three nested squares joined by four spokes, giving twenty-four points where a man may stand. Placing. Players take turns putting one man on any empty point until all eighteen are down. White places first. Moving. After that, a turn means sliding one of your men along a line to an adjacent empty point. You cannot jump, and you cannot move to a point your man does not touch. Flying. Once you are reduced to exactly three men, you may move a man to any empty point on the board instead of only an adjacent one. Three of your men in a line is a mill, and closing one lets you take an enemy man off the board immediately. You win by reducing your opponent to two men, or by leaving them with no legal move at all.

• A line of three only counts along the drawn lines. Three men round a corner is not a mill.
• There are sixteen mills: four on each of the three squares, and four along the spokes.

When you close a mill you take one enemy man — but not one that is standing in a mill of its own, unless every enemy man is in one. That exception matters more than it looks. Without it, a player whose men all sat in mills could never be touched and the game could not end. With it, a player who has carefully protected everything discovers that the protection evaporates the moment it becomes total. Taking is also the only permanent gain in the game. Men never come back. Everything else — a good point, an open line, a cramped opponent — can be undone next turn.

• Check what is protected before you build the mill, not after.
• If all your opponent's men are in mills, the protection lifts entirely — that is often the moment to strike.

Two players who each hold a solid position can shuffle men back and forth forever, because a slide is always reversible. The standard rule stops it: if fifty moves pass for each side with no man taken, the game is a draw. Cognivo counts that as a hundred plies and warns you when the count is close. A position can only repeat while nobody is capturing, so counting quiet moves catches the same shuffle that threefold repetition would — with the advantage that the count is visible to you while it runs.

• If you are ahead on men, watch the counter — a draw throws away a won game.
• If you are behind, the counter is a resource. Shuffling safely is a real plan.

Beginners treat the placing phase as setup and the moving phase as the game. It is the other way round. By the time all eighteen men are down, the shape of the position is fixed and the moving phase largely plays out what placing decided. Two things matter while placing. First, the four midpoints of the middle ring — the points where a spoke meets the middle square. They touch four other points each, more than anywhere else on the board, so a man there has options while a man in a corner has two. Second, do not chase mills. A mill you close during placing takes one man, but if closing it hands your opponent the whole centre you have made a bad trade. Watch what your opponent is building at least as closely as what you are.

• The four middle-ring midpoints have four neighbours each. Corners have two.
• A mill that costs you the centre is usually a bad trade.

A double mill — sometimes a "running mill" — is two mills sharing a man, arranged so that moving that man out of one closes the other. It takes an enemy man every single turn, for as long as it stands, and the opponent can do nothing about it directly. Building one is the clearest winning plan in the game, and preventing one is the clearest defensive priority. The related idea is a double threat: a move that creates two open mills at once, only one of which the opponent can block. This is what the one-ply thinking of a beginner cannot see, and it is precisely what a search finds. If you play the hard opponent here and lose without understanding why, look for the move two turns earlier that made two threats at once.

• Two threats, one blocker. That is the whole game in four words.
• A running mill wins on its own — if you see one coming, spend a move to stop it.

You do not have to reduce your opponent to two men. Leaving them with no legal move wins outright, and against a player who has taken corners and edges without thinking about mobility it is often the faster win. A man on a corner has two neighbours; block both and it is frozen. Freeze them all and the game is over regardless of how many men they still hold. This is why the flying rule exists. A player down to three men can move anywhere, so they cannot be blocked at all — the rule stops the endgame from being a formality and gives the losing side genuine chances. It is also why "reduced to two" and "cannot move" are two separate ways to lose rather than one.

• Count your opponent's available slides, not just their men.
• A player on three men cannot be blocked — they fly. Beat them on count instead.