Technique 6 of 8 · Recognising patterns
Understand the 1–2–1 pattern
The guide and all minesweeper techniques
Minesweeper techniques
- 1Finding certain minesStart here
- 2Finding safe cellsStart here
- 3Flagging and chordingCore technique
- 4Comparing neighbouring numbersShared cells
- 5Subtracting known minesSimplifying clues
- 6The 1–2–1 patternRecognising patterns
- 7The 1–2–2–1 patternRecognising patterns
- 8Using the remaining mine countFinishing a board
In the standard 1–2–1 arrangement, three covered cells sit along one side of three clues. Each end 1 touches its nearest pair; the middle 2 touches all three.
The outer covered cells are mines and the middle is safe. This follows from comparing the groups, not from seeing the digits 1, 2 and 1 anywhere on a board.
Example 1: Two comparisons fix three cells
Step 1 of 3
The left 1 counts the first pair. The 2 needs one extra mine in the right cell.
The left pair contains one mine. The middle 2 includes that pair plus the right cell, so the right cell is a mine.
From the other direction, the right pair supplies one of the 2's mines, forcing the left cell to be a mine. With both ends accounted for, the middle cell is safe.
Work inside the solid frame. Every possible mine neighbour of these clues is inside it. Cells beyond the frame are background, not revealed zeros or extra clues.
Flags are proved mines. A tick marks a cell proved safe; its number is not shown.
Blue-ringed numbers are the clues to compare. Shading marks the cells being discussed.
Example 2: The digits alone are not enough
Step 1 of 3
Check the surroundings before applying the pattern. This time both sides of the numbers are covered.
Here there are covered cells on both sides of the clue row. Each end group must still contain one mine, but the mine can sit above or below.
Both middle cells are safe. The end cells are not individually fixed: flagging every outer cell as if this were a single covered row would be wrong.
Work inside the solid frame. Every possible mine neighbour of these clues is inside it. Cells beyond the frame are background, not revealed zeros or extra clues.
Flags are proved mines. A tick marks a cell proved safe; its number is not shown.
Blue-ringed numbers are the clues to compare. Shading marks the cells being discussed.
Try it yourself
Try the reasoning yourself. Choose an answer to see the explanation.
The standard arrangement has been turned sideways. Use the same group comparisons to find its safe cell.
Practice only: no clock, saved game or hint charge.
Task 1 of 1: Choose the cell proved safe.
Choose a cell on the board.
Work inside the solid frame. Every possible mine neighbour of these clues is inside it. Cells beyond the frame are background, not revealed zeros or extra clues.
Practice close-up: rows 1–4, columns 1–4.
Flags are proved mines. A tick marks a cell proved safe; its number is not shown.
How to spot it faster
- Check which covered cells each end 1 touches.
- Turn or reflect the whole arrangement; the groups matter more than its orientation.
- Reduce clues by proved flags before deciding whether the remaining groups match.
Common mistakes
Flagging by the digits alone
Extra covered neighbours can change which individual cells are fixed. Trace the full groups, including both sides of the clue row.
Treating the middle 2 as a mine
A revealed 2 is safe. The pattern describes covered neighbours beside the clue row.
Questions
Further reading
Try it
Look for a 1–2–1 beside an opening. Check its full covered neighbourhood, then explain the mine at each end before opening the middle.
Play solo Minesweeper