Sudoku solving technique · Pro
Skyscraper
An almost X-Wing: two base cells share a column, the two roofs do not.
What is a Skyscraper?
A Skyscraper uses a candidate that appears in exactly two cells in each of two rows. The two base cells, one from each row, share a column; the two roof cells lie in different columns. The base cells cannot both hold the candidate, so at least one roof does. It can be eliminated from any cell that sees both roofs. The same works with columns instead of rows.
How to spot a Skyscraper
Start as you would for an X-Wing, with a digit that fits in only two cells in each of two rows. If the columns only half match, so that just two of the four cells share a column, those two are the base and the other two are the roofs. Remove the digit from cells that see both roofs. The same works with columns instead of rows.
- Difficulty
- In Sudoku Pro, this technique belongs to the Pro level.
- Learning path
- Technique 10 of 20 on the learning path
An example from the app
| Pattern (purple). Candidates 2, 4 | Given 3 | Given 1 | Pattern (purple). Candidates 2, 4 |
| Empty | Empty | Affected cell (orange). Candidates 3. Eliminated 4 | Empty |
| Empty | Empty | Given 2 | Affected cell (orange). Candidates 3. Eliminated 4 |
| Pattern (purple). Candidates 3, 4 | Given 2 | Pattern (purple). Candidates 3, 4 | Given 1 |
Small digits are candidates. Crossed-out digits can be removed in this step.
- Pattern (purple)
- Affected cell (orange)