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Minesweeper 1-2-2-1 Pattern

Four covered squares sit above 1-2-2-1. Call them A, B, C and D from left to right. The middle two have mines. Both ends are safe. Compare two numbers at a time to see why.

Try this pattern

See the pattern step by step

Four covered squares, A through D, sit above 1, 2, 2 and 1. Each end 1 touches two covered squares. A + B = 1 and A + B + C = 2 make C a mine. The right 1 makes D safe. The right 2 then makes B a mine, so A is safe. Flags mark mines in B and C in the middle. The end squares, A and D, are open and each shows 1. The clue row stays 1-2-2-1.
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Board example

A and B have 1 mine in total. A, B and C have 2 mines. The extra mine must be in C.

Before

Four covered squares, A through D, sit above 1, 2, 2 and 1. Each end 1 touches two covered squares.

After

Flags mark mines in B and C in the middle. The end squares, A and D, are open and each shows 1. The clue row stays 1-2-2-1.

Why it works

C has a mine, so the right 1 makes D safe. The right 2 counts two mines in B, C and D. C is a mine and D is safe, so B has the other mine. The left 1 then makes A safe. In the picture, A and D both open to show 1.

A + B = 1 and A + B + C = 2 make C a mine. The right 1 makes D safe. The right 2 then makes B a mine, so A is safe.

The rule

The left 1 counts mines in A and B. The next 2 counts mines in A, B and C. The next 2 counts mines in B, C and D. The right 1 counts mines in C and D. These are the only unknown squares around the numbers. Every other nearby square is safe.

What to check

Check every nearby square, including the corners and the other side. A, B, C and D must be the only unknown squares. Subtract any known mines from the numbers they touch. Apply the pattern when the remaining counts are 1-2-2-1.

Subtract a mine from the clues it touches

Here the clue row is 2-3-2-1. A known mine below the first clue touches only the first two clues. Subtract 1 from those two numbers. Leave the other counts as they are. The remaining row is 1-2-2-1, so the middle two covered squares above have mines and both end squares are safe. The board still shows 2-3-2-1.

Try this pattern

Use the numbers to choose your next move. Include the corners when you count. Squares outside the picture are open. Each flag marks a known mine.

Select every square you can prove safe.

1221

See why

C has a mine, so the right 1 makes D safe. The right 2 counts two mines in B, C and D. C is a mine and D is safe, so B has the other mine. The left 1 then makes A safe. In the picture, A and D both open to show 1.

Read the board, then choose

Adding a known mine below the first pair changes 1-2-2-1 into 2-3-2-1. Subtract that mine from the two numbers it touches to get 1-2-2-1 again. The two middle covered squares have mines.

Adding a known mine below the first pair changes 1-2-2-1 into 2-3-2-1. Subtract that mine from the two numbers it touches to get 1-2-2-1 again. The two middle covered squares have mines.

Choose a covered square. Then decide whether it is safe, has a mine, or needs more information.

A
B
C
D
Why this answer is correct

A: Safe · B: Mine · C: Mine · D: Safe

The shown covered squares are the only unknown squares around these numbers. Gaps stand for open squares or the board edge. Check every nearby square before you apply the result. Flags in these examples mark known mines.

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