GridWits
Hard tier Technique 10 of 46 Used by Smart Hint

Hidden Quad

Four digits confined to the same four cells in a unit — strip those cells down to just those four digits.

How it works

  1. 1Row 7 has no digits placed yet, so all nine cells are still open — and that is exactly what keeps a cheaper subset out of reach.
  2. 2Digits 2, 4, 6 and 9 appear only in A7, C7, E7 and G7. The five cells left over share five digits between them, so they form no pair, triple or quad.
  3. 3Those four digits must fill those four cells, so their extras go: A7 loses 1, C7 loses 3, E7 loses 5 and G7 loses 7.

Play a Sudoku that needs this technique →

A 1 B 2 C 3 D 4 E 5 F 6 G 7 H 8 I 9 1 2 4 1 3 8 3 4 6 3 5 7 5 6 9 1 5 8 2 4 7 9 3 7 8 1 5 7 2 4 4 6 6 9 2 4 9 1 3 5 7
  • Cells that establish the pattern
  • Candidate ruled out
Next in the ladder X-Wing →

A digit's candidates in two rows fall in the exact same two columns, forming a rectangle — eliminate it from the rest of those columns.