Square of Four
Four equal numbers in a 2×2 square can only be rejected one diagonal or the other. Tucked into a corner of the board, the choice is forced: rejecting the two cells beside the corner would seal the corner cell off, so the corner and its diagonal partner are the rejected ones.
How it works
- 1Four 4s fill a 2×2 square tucked into the board's corner. In each of the square's rows and columns the two 4s sit side by side, so exactly one of each pair is rejected — the square always resolves to one rejected diagonal or the other.
- 2Try the diagonal that keeps the corner cell: its only two neighbours are the square's other diagonal, and both would be rejected. The kept corner would be sealed off from every other kept cell — the rules forbid that.
- 3So it must be the other diagonal: reject the corner and its diagonal partner, keep the two beside them. Away from a corner, a square of four stays open until something else decides one of its cells — then the basic rules finish it.
- Kept
- Rejected
- Cells that establish the pattern
- Where the conclusion lands