Minesweeper 1-2-2-1 Pattern — Full Deduction

The 1-2-2-1 pattern is one of the cleaner configurations in Minesweeper. Four numbers in a row along the edge of cleared territory, with unknown cells on one side. When it appears, two mines are always in specific positions — and two cells are always safe to click.

What the pattern looks like

Four consecutive numbered cells in a row: 1, 2, 2, 1. The unknown cells are on one side (above, below, left, or right depending on orientation). Along the edge, nothing else is unknown — only the row directly adjacent to the 1-2-2-1 sequence.

The deduction

Label the unknown cells A, B, C, D from left to right, directly adjacent to the 1, 2, 2, 1 respectively.

Work through each number:

• The first 1 has only cells A and B as unknown neighbors. It needs exactly 1 mine between them.
• The last 1 has only cells C and D as unknown neighbors. It needs exactly 1 mine between them.
• The first 2 neighbors A, B, and C. It needs exactly 2 mines among those three.
• The last 2 neighbors B, C, and D. It needs exactly 2 mines among those three.

From the first 1: exactly 1 mine in {A, B}.
From the first 2: exactly 2 mines in {A, B, C}. Since {A, B} already has 1 mine, C must be a mine.
From the last 2: exactly 2 mines in {B, C, D}. Since C is a mine, {B, D} must together have 1 more mine.
From the last 1: exactly 1 mine in {C, D}. Since C is a mine, D must be safe.

Substituting back: D is safe. From the last 1 constraint, C holds the mine for that number. From the first 1, exactly one of A or B is a mine.

The actual conclusion

• C is a mine — certain
• D is safe — certain (click it)
• A is safe — certain (click it)
• B is the remaining mine — certain (flag it)

Here's why A is safe: From the first 1, exactly 1 mine in {A, B}. We now know C is a mine and B must cover the mine for the last 2. If B is the mine satisfying the first 2 and last 2, then A is safe. Let's verify:

• First 1: 1 mine in {A, B} → B is the mine ✓
• First 2: 2 mines in {A, B, C} → B and C ✓
• Last 2: 2 mines in {B, C, D} → B and C ✓
• Last 1: 1 mine in {C, D} → C ✓

Everything checks out. A is safe, B is a mine, C is a mine, D is safe.

Rotations and reflections

The 1-2-2-1 pattern is symmetric — reflected left-to-right, it's the same analysis with A and D swapped, B and C swapped. The mines are always in the two middle positions (B and C). The corner cells (A and D) are always safe.

The pattern works in all four orientations — horizontal along the top edge, bottom edge, or vertical along either side. The unknowns are always on the open side.

The 1-2-1 pattern, for comparison

Three numbers in a row. The middle 2 has three unknown neighbors, but the outer 1s each constrain the ends. The middle cell is always a mine; the outer cells are always safe. Same logic, shorter sequence. Our 1-2-1 guide covers it in detail.