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Minesweeper: When Every Hidden Square Is a Mine

The all-hidden-are-mines technique says: when a number touches exactly as many hidden squares as mines it still needs, every one of them is a mine. Together with all mines found, it makes most of the moves in any game.

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This is the second of the Minesweeper techniques.

The rule

  1. Pick a number on the edge of the open ground.
  2. Take off the flags already around it: what’s left is how many mines it still needs.
  3. Count its hidden neighbours. If that’s the same number, flag them all.
All hidden are mines: a 2 in the corner A row of hidden squares over a row of numbers. The 2 at the right end touches only two hidden squares, so both are mines. 1 1 1 1 2 2 2
The ringed 2 touches only two hidden squares, so both are mines.

In a real game

On the Expert board every technique page follows, it’s the very first move. The first tap opened 53 squares, and on the edge of them, a 1 touches just one hidden square. That square is its mine.

Step 1: all hidden are mines Part of the Expert board after the first tap. The ringed 1 touches just one hidden square, so that square is its mine. 3 1 3 1 1 1
Step 1: the ringed 1 has one hidden neighbour, so it’s a mine.

How often you’ll need it

Almost always: on 199 of 200 Beginner boards, 194 Intermediate and 190 Expert. On the Expert board here it makes 77 of the 215 moves.

Next steps

Questions

Do I have to flag the mines it finds?

No, but it helps. Flagged mines make the next all-mines-found check easy to see, and you need them flagged to chord.

What about flags already next to the number?

Take them off first. A 3 with one flag beside it still needs two mines, so it’s two hidden squares you’re looking for, not three.