Step 18 · Master
What is Skyscraper? How to Solve Killer Sudoku
When a digit has only two spots in each of two rows and one end of each lines up in the same column, you can remove that digit from cells that see both of the other two ends.
What it is
Think about one digit (say 5). Two rows each have only two spots for 5, and one end of each lines up in the same column — like two buildings standing side by side.
The two lined-up cells share a column, so they can't both be 5. That means one of the other ends (the rooftops) must be 5.
Any cell that sees both rooftops can't be 5.
How to spot it
Pick one digit and find two rows (or columns) where it has exactly two spots. If only one end lines up, it's a Skyscraper.
Example
Dotted outlines are cages, with their total in the top-left corner. Yellow cells are where to look, orange cells are the reason, red digits are candidates you can remove, and the green digit is the answer.
Think about 7 only. row 2 and row 8 each have just two spots for 7. R2C8 and R8C8 line up, so they can't both be 7. That means one of R2C7 and R8C9 must be 7. Any cell that sees both (R3C9 and R7C7) can't be 7.