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The double mill (Zwickmühle)
In Nine Men's Morris the strongest position is not a big army. It is five men arranged so that one of them can walk back and forth and close a mill every time it moves. German players call it the Zwickmühle. Once one is running, the other side loses a man every move.
What it is
You have a man that can step between two points, and at each point it completes a different line of three. Step out, and one mill is closed. Step back, and the other is. Each move takes an opponent's man, with no setup in between. The German dictionary Duden defines it as a position in which you get a new mill every time by pushing one stone back and forth.
The word has a second life in everyday German. In der Zwickmühle sein means to be in a bind with no way out, which tells you how the loser feels. English players say double mill, and the English Wikipedia article on the game describes the winning structure as a "double-mill" shuttle.
The simplest shape
Four anchor men and one shuttle, five in all, laid out on two neighbouring squares as in the picture. The anchors sit on the two top corners of the outer square and the two top corners of the middle square. The shuttle stands on the middle of the outer top side. When it is there, it closes the outer top row. Sliding it along the line to the middle of the middle square's top side closes the middle row. Back again, and the first row closes again.
Why is this so hard to stop? The outer point of the shuttle has three neighbours, and two are your own anchors and the third is the point you shuttle to. The opponent cannot step into it once you leave. The middle point has one extra neighbour, the top of the inner square. That is the only door. If an opposing man stands there when the shuttle goes out to the rim, it can step into the middle square, and the shuttle is stuck on the rim.
Capturing is your other weapon. While the shuttle is on the rim, the outer row is a closed mill and its men are protected, but the middle row anchors are not. Which men you take with your first captures matters: usually the man on the inner top point (the door), then any man that could become a mill of his own.
How to build one
- Plan the anchors early but place them late. Two men at the ends of a side with the middle empty is an open threat, and your opponent will fill the middle. Better that the middle is yours (the shuttle) before the second pair of anchors arrives.
- Bring the last anchor with tempo, as a move that also threatens something else, so your opponent has to answer there and cannot spare a move for the door.
- Count the door. Before you start, look at the single point next to the shuttle's second stop. If it is empty and your opponent has no man within a move of it, you are fine.
- Be patient in the moving phase. A double mill is often built from men that already stand around; sliding one of them is quieter than placing a new man where everyone sees it.
How to break one
If your opponent is building a double mill, you have these choices, roughly in the order I would try them.
- Occupy a stop of the shuttle, or stand a man on the door so you can step into the stop the moment it is vacated.
- Take an anchor. You can only do that if you close a mill yourself, and only anchors outside a closed mill are open to capture. At any moment, that is the anchors of the row that is not currently closed.
- Make your own threats so fast that he has no time to shuttle. A threat he must answer costs him a move.
- If you are down to three men, fly. A flying man can land on the vacated stop at once, which is why a double mill does not finish a flyer so easily.
Other shapes
Any pair of adjacent points will do, as long as each completes a different line, so the double mill appears in many forms. Another shape runs along the rim: one stop in the corner of the outer square, where it closes the top row, and the next stop on the right-hand side, where it closes the spoke towards the centre. It needs the same four anchors and five men in all, and it has one door, the next point down the right-hand side.
The twelve-man board of MorrisTalk adds diagonal lines, which creates still more of them.
Read the strategy guide for the neighbours of a double mill (forks, blockades), the opening guide for how to get your men there, and the flying rule for why flyers are hard to shuttle against.
Frequently asked questions
What is a Zwickmühle in English?
A double mill: a position where one man can move back and forth between two points and close a different mill each time, so every move takes an opponent's man.
How many men do you need for a double mill?
Five in the simplest shape: four fixed anchors, two on each of two parallel rows, and one man shuttling between the rows.
How do you stop a double mill?
Put a man on a point the shuttle needs, keep a man next to the point it vacates, take one of the anchors with a mill of your own, or, with three men, fly into the vacated point.