The Skyscraper Technique in Sudoku
The Skyscraper is a single-digit technique that is often easier to see than the X-Wing. You find two lines where a digit has just two spots, sharing one “base” line — the two far ends form towers, and any cell that sees both towers cannot hold the digit. It is a quick, reliable way to break a stalled hard grid.
How to use the Skyscraper, step by step
- Chase one digit and find two rows where it has exactly two candidate cells each.
- Check that one pair of those cells shares the same column — that shared column is the “base”.
- The two cells in different columns are the “tops” of the towers.
- Find any cell that can see both tower tops at once.
- Eliminate the digit from those seeing cells — one tower top must hold it, so a cell seeing both cannot.
- Re-scan for the singles this opens up.
What is a Skyscraper?
A Skyscraper is a single-digit pattern: two lines (say two rows) where a digit has only two candidates each, with one candidate from each sharing a column — the base — leaving two “tower top” cells elsewhere. Because the digit must sit at one tower top or the other, any cell that sees both tops can have the digit removed.
How to spot a Skyscraper
Look for two rows (or two columns) where one digit has exactly two spots, and where one spot from each lines up in a shared column (or row). The other two spots are your towers. It is closely related to the X-Wing — an X-Wing needs both ends aligned, a Skyscraper only needs one — which makes the Skyscraper more common and often quicker to find.
- Work from full pencil marks.
- Check both the row version and the column version. See the full set in tips & strategy.
Why the elimination works
One of the two tower tops must hold the digit, because the base column can take it on only one of its two lines. So any cell that can see both tops is guaranteed to have the digit as a neighbour, and can never hold it itself. Pure deduction, no guessing.
Skyscraper in Sudoku: FAQ
What is a Skyscraper in Sudoku?
A Skyscraper is a single-digit technique using two lines where a digit has two candidates each, sharing one column or row as a “base”. The two unaligned cells are towers; any cell that sees both towers cannot hold the digit, because one tower must.
How is a Skyscraper different from an X-Wing?
An X-Wing needs the digit aligned in the same two columns on both rows. A Skyscraper only needs one shared column, with the other two cells free — so it is more common and often easier to spot, while eliminating from cells that see both tower tops.
How do you spot a Skyscraper?
Chase one digit, find two rows where it has exactly two candidates, and check whether one cell from each shares a column. If so, the other two cells are towers, and any cell seeing both can have the digit removed.
When should you use the Skyscraper?
It is a hard-grid technique, used once singles and pairs stall. Because it is easier to see than many fish, it is a good first single-digit pattern to try on Expert and Evil puzzles.