Sudoku techniques · Expert
Skyscraper in Sudoku
A Skyscraper takes two strong links on the same number, meaning two lines where the number has only two possible cells, and lines up one end of each in the same row or column. Those two ends are the floor; the other two are the roof. At least one roof cell must hold the number, so anything seeing both roofs loses it.
Why it works
When two strong links on one number have floor ends that share a row or column, at least one roof end must hold it, and any cell seeing both roofs loses that candidate.
How to spot it
Think of it as a broken X-Wing, and that is exactly what it is.
- Find two rows where a number has exactly two spots each: the same start as an X-Wing.
- If the four cells do not share two columns but two of them do share one column, that shared column is the floor and the pattern still works.
- The eliminations sit where the two roof cells' boxes and lines overlap.
Builds on
Worked example
These are the app's own lesson boards. Step through them the same way a hint does.
- 1
In each of the two highlighted columns, 3 can only go in two cells (a strong link). The two cells marked (the floor) share one row. The two cells marked (the roof) stand in two different rows. This is a Skyscraper!
- 2
3 must be in at least one of the two brighter cells , so we can remove the candidate 3 from any other cell that sees both of them.