The Skyscraper
Learn the Skyscraper: two strong links of one digit that share a base line, letting you eliminate the digit from any cell that sees both far ends.
A Skyscraper is the easiest single-digit chain to learn. Find a digit that appears exactly twice in each of two rows, where one position from each row lines up in the same column. That shared column is the "base", and the other two cells are the "roof".
Because each row is a conjugate pair, one roof end or the other must hold the digit. So any cell that can see both roof cells cannot be the digit, and you remove it there. Swap rows and columns for the mirror version.
It's the simplest member of the turbot-fish family and a good first step into chain logic. Base, roof and the eliminations the roof forces are all marked in the example.
Practise the Skyscraper
The best way to learn a technique is to use it. Play a puzzle at the level where it first appears, or drop a tricky board into the solver to watch it in action.
Frequently asked questions
What is a Skyscraper in Sudoku?
Two conjugate pairs of a digit in two lines that share one base column (or row). One of the two far "roof" ends must be the digit, so cells seeing both roofs lose it.
Is a Skyscraper the same as an X-Wing?
No. An X-Wing needs both pairs aligned in the same two columns; a Skyscraper has them aligned on only one line, with the other ends offset. The Skyscraper eliminates via cells that see both roof ends.
When do I use a Skyscraper?
On hard and expert boards when singles, pairs and the basic fish stall. It is usually easier to spot than an XY-Wing.
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