Minesweeper patterns: understand 1-2-1 and 1-2-2-1

A Minesweeper pattern is a shortcut for a deduction you understand. Two useful examples are 1-2-1 and 1-2-2-1. They work because neighbouring clues overlap, but the digits alone are not enough: the hidden cells touching them must also match.

The 1-2-1 pattern

Suppose three opened clues, 1, 2, 1, sit above three hidden cells A, B and C. The left 1 touches A and B. The middle 2 touches A, B and C. The right 1 touches B and C.

121ABC

The left 1 tells you that A and B contain one mine between them. The middle 2 sees the same pair plus C and needs two mines, so C must be a mine.

The right 1 now has its mine in C. That makes B safe. Finally, the left 1 still needs a mine, so A must be a mine.

121M✓M

You do not have to memorise the final picture. You can recover it by comparing the left pair with the middle group. This is a useful habit when the board is rotated or the same pattern appears vertically.

The 1-2-2-1 pattern

Now take four clues, 1, 2, 2, 1, above four hidden cells A, B, C and D. The left 1 touches A and B. The first 2 touches A, B and C. The second 2 touches B, C and D. The right 1 touches C and D.

1221ABCD

Compare the left 1 with the first 2. A and B already supply one mine, so C is a mine. Compare the right 1 with the second 2. C and D supply one mine, so B is a mine.

With B known, the left 1 is satisfied and A is safe. With C known, the right 1 is satisfied and D is safe.

1221✓MM✓

When these patterns do not apply

Do not flag cells just because you see 1-2-1 written somewhere on a board. An extra hidden cell above a clue, beside the row or diagonally around an end changes the possible mine positions. A jagged boundary can look like the pattern without having the same neighbourhoods.

Likewise, a proved mine touching a clue changes how many mines it still needs. A displayed 2 next to one proved mine has a remaining count of 1. You may be able to reduce a larger set of clues to a familiar pattern, but first write down the remaining counts and the exact unknown neighbours.

A flag you have not justified is especially dangerous here. The reasoning depends on known mines, not on symbols placed as guesses.

A reliable way to check a pattern

1. Choose the clue row and identify every hidden neighbour of each clue.
2. Account for any mines already proved around those clues.
3. Check that the remaining counts and shared-cell groups match the example.
4. Explain the deduction before making the move.
5. Re-read the new numbers after the safe cell opens; the next deduction may be different.

If the shape does not match, return to the basic rules in what Minesweeper numbers mean. Finding one simple safe cell is more useful than forcing a pattern onto the wrong part of the board.

Using patterns in Sweep

Sweep uses the same adjacent-mine counts. Its regular levels do not use chording to open a clue’s neighbours all at once, so make each proved move with a tap. The FLAG button starts at level 3. If you need a first lesson before combining clues, read the beginner’s guide.