Hard Used from hard puzzles up.
Skyscraper
Two strong links on one base
Pick a digit with exactly two places in each of two rows. In an X-Wing they line up in the same two columns; in a Skyscraper only one place of each row lines up, in one column: the base. The other two places, out of line, are the tops.
The two base cells see each other, so they are not both the digit. If the first row's base cell is not the digit, that row's top is. If it is, the other row's base cell is not, and that row's top is. Either way, one of the two tops holds the digit.
So a cell that sees both tops cannot hold it: remove it there. It works the same with columns and rows swapped. A Skyscraper is the shortest X-Chain, four cells long.
What to look for
Pick a digit and look for two rows (or columns) where it has two places each, one of each in the same column (or row).
Example
- The cells the technique relies on
- Cells where it removes candidates, struck out in red
- Strong link (solid line) and weak link (dashed)
Practise it
In the app, Learn › Skyscraper sets exercises on real positions, then a full puzzle that needs this technique and nothing harder.