Skyscraper
Prerequisites: X-Wing
Overview
A Skyscraper uses one digit with exactly two possible cells in each of two rows (or two columns). One end of each pair lines up with the other, forming the base; the other ends are the tops. At least one top holds the digit, so it can be removed from cells that see both tops.
How It Works
Find a digit with exactly two possible cells in each of two rows (or columns). If one cell from each pair shares a column (or row), those two are the base and the other two are the tops. The base cells see each other, so at most one of them holds the digit; the other pair's top must then hold it. So at least one top holds the digit, and any cell that sees both tops can't. If the tops lined up too, it would be an X-Wing.
When To Use It
Look for a Skyscraper when a digit has exactly two places in each of two parallel rows or columns and one place from each pair lines up. It is an X-Wing with one leg shifted.
How To Detect It
For each digit, find rows (or columns) where it has exactly two places. Take two of them and check whether one place from each shares a column (or row): that is the base. The other two are the tops. Remove the digit from every cell that sees both tops.
Visual Example
Step 1 of 8
One Digit, Two Columns
A Skyscraper works with a single digit that has exactly two possible cells in each of two columns (or two rows). In each column, one of the two cells must hold the digit: a strong link.
Candidates already ruled out by earlier techniques aren't shown.
Practice
Your goal: fill the highlighted cell. Select a cell, then pick a number.
A Skyscraper on one digit is hiding here. Use it to eliminate, then fill the Naked Single that results.Candidates already ruled out by earlier techniques aren't shown.
Use it in a real game
Hard puzzles are where you'll need techniques like this one.
Play a Hard puzzle