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Minesweeper 1-2-1 Pattern

Three covered squares sit above 1-2-1. Call them A, B and C from left to right. Both ends have mines. The middle is safe. Follow the numbers to see why.

Try this pattern

See the pattern step by step

Three covered squares, A, B and C, sit above 1, 2 and 1. These are the only unknown neighbors of the clues. The 2 counts A, B and C. The left 1 counts A and B, so C is a mine. The right 1 counts B and C, making B safe and A a mine. Flags mark mines in A and C at both ends of the top row. B in the middle is open and shows 2. The row below stays 1-2-1.
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Board example

A and B have 1 mine in total. A, B and C have 2 mines. B and C have 1 mine in total.

Before

Three covered squares, A, B and C, sit above 1, 2 and 1. These are the only unknown neighbors of the clues.

After

Flags mark mines in A and C at both ends of the top row. B in the middle is open and shows 2. The row below stays 1-2-1.

Why it works

Suppose B has a mine. Both 1s would then make A and C safe. That leaves only one mine for the 2. So B is safe, and A and C have mines. In the picture, B opens to show 2.

The 2 counts A, B and C. The left 1 counts A and B, so C is a mine. The right 1 counts B and C, making B safe and A a mine.

The rule

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

What to check

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

Find 1-2-1 inside a row of 2-3-2

A known mine below the middle of 2-3-2 touches all three clues. Subtract it from each: 2 − 1 = 1, 3 − 1 = 2, and 2 − 1 = 1. The three covered squares above now have the remaining counts 1-2-1. Both ends have mines, and the middle is safe. These remaining counts are notes for your reasoning. The numbers printed on the board stay 2-3-2.

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.

121

See why

Suppose B has a mine. Both 1s would then make A and C safe. That leaves only one mine for the 2. So B is safe, and A and C have mines. In the picture, B opens to show 2.

Read the board, then choose

Each flag marks a known mine. Subtract it from every number it touches: 2 − 1 = 1, 3 − 1 = 2, 2 − 1 = 1. The remaining counts are 1-2-1. In the covered row, both ends have mines and the middle is safe.

Each flag marks a known mine. Subtract it from every number it touches: 2 − 1 = 1, 3 − 1 = 2, 2 − 1 = 1. The remaining counts are 1-2-1. In the covered row, both ends have mines and the middle is safe.
Subtract the known mines

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

A
B
C
Why this answer is correct

A: Mine · B: Safe · C: Mine

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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