1-2-1 pattern

Level: Advanced

A 1-2-1 along a wall confirms both ends as mines.

This is a classic pattern where 1-2-1 appears in a row along a straight border of an open area. Overlaying the candidate cells each number shares pins the arrangement down to one.

In a typical 1-2-1 arrangement, the cells below each end 1 (outside the border) are confirmed as mines, while the cell below the middle 2 is safe. It resolves especially cleanly when your back is to the wall.

It shows up so often that it's worth memorizing outright. But if other numbers are entangled nearby, exceptions can arise, so always check the adjacency conditions before applying it.

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