Swordfish

A worked example

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

1. Where to look

   Filter to one digit. Count the base lines and the union of the crossing lines.

   3 base rows confine a digit to 3 cover columns (or transpose rows and columns). Those columns are reserved for the bases.

2. Read the evidence in this position

   For 7, the base houses are column 1, column 7, column 9. The covers are row 2, row 3, row 8.

3. Why the deduction must follow

   Each of the 3 base lines needs one 7. All 3 copies must fit into these 3 cover lines, without any two sharing a cover. So every cover is occupied by a base-line 7. No other 7 fits outside the bases.

4. Apply the result

   Erase 7 from r8c2. The pattern may remove candidates without immediately solving a cell.