Step 13 · Master
What is Skyscraper? How to Solve 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
Yellow cells are where to look, orange cells are the reason, red digits are candidates you can remove, and the blue digit is the answer.
Think about 2 only. row 3 and row 7 each have just two spots for 2. R3C3 and R7C3 line up, so they can't both be 2. That means one of R3C4 and R7C6 must be 2. Any cell that sees both (R1C6 and R9C4) can't be 2.