Technique 4 of 8 · Shared cells
Compare what two numbers are counting
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
Two numbers often count some of the same covered cells. Instead of trying to place each mine separately, compare what the two groups have in common and what is left over.
If a 1's entire group is inside another 1's group, the extra cells in the larger group are safe. Both clues already count the same one mine, even if you do not know which shared cell holds it.
Example 1: The same mine, with extra cells
Step 1 of 3
The left 1 counts exactly one mine in the two shaded cells.
The left 1 counts the two cells above it. The right 1 counts those same two, plus the top-right and bottom-right cells.
The shared pair contains exactly one mine. That uses the right 1's whole allowance, so both extra cells are safe. We can make progress without deciding which cell in the shared pair is the mine.
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: One extra mine in one extra place
Step 1 of 3
The left 1 says the first pair contains one mine.
Here the tick is a safe cell proved earlier. The left 1 counts the first two covered cells, and the 2 counts those same two plus the third.
The shared pair contributes one mine. The 2 still needs one more, and only the third covered cell is outside that pair. That cell must be a mine.
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.
Compare the two 1s. You do not need to locate their shared mine to find the safe cells.
Practice only: no clock, saved game or hint charge.
Task 1 of 1: Choose both cells made safe by the larger group.
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
- Trace each clue's full group before subtracting; a diagonal can change which group contains the other.
- Ask what the extra cells must contribute, rather than where every mine is.
- Equal requirements make the extra cells safe. A difference only identifies individual mines when every extra cell is needed.
Common mistakes
Comparing numbers without comparing cells
Two adjacent 1s do not automatically prove a safe cell. One clue's remaining group must fit entirely inside the other's for this subtraction.
Splitting a mine between unknown cells
A pair containing one mine does not tell you which member is the mine. Keep that uncertainty until another clue resolves it.
Questions
Further reading
Try it
Look for two nearby numbers with overlapping covered neighbours. Name the shared group and the extra cells before making a move.
Play solo Minesweeper