Pointing pair

A worked example

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

1. Where to look

   Start inside a box. Check whether all copies of one candidate line up.

   A box needs a digit, and its only two possible locations are on one line. That line cannot contain the digit anywhere outside the box.

2. Read the evidence in this position

   In box 1, all remaining 7 candidates are r2c1, r3c1. All are also in column 1.

3. Why the deduction must follow

   box 1 must get its 7 from these cells. Wherever that 7 lands, column 1 already has its one 7. Its other cells outside box 1 therefore cannot contain 7.

4. Apply the result

   Erase 7 from r7c1, r8c1. The pattern may remove candidates without immediately solving a cell.