Skyscraper

A worked example

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

1. Where to look

   Find two parallel strong links and align one pair of endpoints.

   Two two-location rows or columns have linked near ends. At least one far end must hold the digit.

2. Read the evidence in this position

   Trace r3c42 → r3c52 → r9c52 → r9c42. Solid links mean at least one end is true; dashed links mean the two ends cannot both be true.

3. Why the deduction must follow

   Assume the first candidate is false. The next is forced true, the next false, and so on, ending true. If the first candidate was true instead, it already supplies the required 2. At least one endpoint is 2; a target seeing both endpoints cannot be 2.

4. Apply the result

   Erase 2 from r4c4, r6c4. The pattern may remove candidates without immediately solving a cell.