Hidden quad

A worked example

This position comes from SudokuByte’s lesson on the Hidden quad. Gold cells hold the evidence; green cells are the pattern’s ends.

1. Where to look

   Track where selected digits can go, rather than looking for small candidate lists.

   4 digits are restricted to the same 4 cells in a house. Those cells cannot contain other digits.

2. Read the evidence in this position

   In row 3, the digits 1, 4, 6, 7 appear only in r3c1, r3c3, r3c7, r3c9.

3. Why the deduction must follow

   Those 4 digits need 4 different cells. These are their only 4 cells, so those cells have no room for an extra digit. Remove the other pencil marks inside them.

4. Apply the result

   Erase 2 from r3c1, r3c9. Erase 3 from r3c7, r3c9. Erase 5 from r3c1, r3c3, r3c9. The pattern may remove candidates without immediately solving a cell.