Naked quad

A worked example

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

1. Where to look

   Compare candidate lists in cells that share a house. Count the union of their digits.

   4 cells in one house contain only 4 different candidates between them. They reserve those digits, in some order.

2. Read the evidence in this position

   In row 7, r7c1, r7c3, r7c6, r7c8 collectively contain only 1, 3, 4, 9. Check every pencil mark in these cells.

3. Why the deduction must follow

   These 4 cells need 4 different values, chosen from exactly 4 digits. They therefore use all those digits, in some order. Remove those digits from the other cells of the house.

4. Apply the result

   Erase 1 from r7c4. Erase 3 from r7c4. Erase 4 from r7c4. Erase 9 from r7c4. The pattern may remove candidates without immediately solving a cell.