Skyscraper
Two lines sharing a base but with tops in different columns. Cells seeing both tops can drop the digit.
What it is
A Skyscraper uses two rows in which a digit has exactly two candidates. One pair of cells shares a column — the base — while the other two cells sit in different columns, forming the tops. At least one of the two tops must contain the digit, because the base cannot hold it twice. Any cell that sees both tops can therefore never hold the digit.
How to spot it
Find two rows where the digit has exactly two candidates each, with one candidate column shared. The two unshared cells are the tops. Then look for cells that share a unit with both tops — usually where their rows and boxes intersect — and remove the digit there.
Worked example
Step through the reasoning below. Each step adds one layer: first the position, then the pattern, then the candidates involved, and finally what can be eliminated.
Step 1 of 4: The starting position
- Pattern cells
- 5/4, 5/8, 9/6, 9/8
- Digits
- 7
- Eliminations
- 4/6, 8/4
Press a marked square to point it out on the board above.
Why it matters
The Skyscraper is the most approachable of the single-digit chain patterns and a natural next step after the X-Wing. It applies in many positions where an X-Wing just fails, and the reasoning is short enough to verify at the board.
Frequently asked questions
How does a Skyscraper differ from an X-Wing?
In an X-Wing all four cells align in two columns. In a Skyscraper only one column is shared; the other two cells sit in different columns, so the eliminations happen where those two cells overlap instead of along whole lines.
Which cells can I actually eliminate?
Only cells that see both tops — that is, share a row, column or box with each of them. Everything else stays untouched.
Why must one of the tops be true?
Because the two base cells lie in the same column and cannot both hold the digit. Whichever base cell is false forces its row's other candidate — a top — to be true.
Put it to use
Where you will meet it
Hard grids are built around moves like this one. That is where it earns its keep.
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