Fish

Skyscraper

A skyscraper is two rows that each hold a digit in exactly two cells, where one of those cells lines up in the same column in both rows. That shared column is the base. Only one of the two base cells can take the digit, so at least one row is forced into its other cell: the two cells at the far ends can't both be empty, and the digit can be removed from anything that can see both of them at once.

Written by Georgi Nedyalkov

Level
Advanced

The pattern

Skyscraper: what it does.

It's a fish that has been bent. An X-Wing needs both rows to share both columns; a skyscraper needs them to share one, and gets its elimination from the two cells that don't line up.

The removal is weaker than an X-Wing's and easier to miss: instead of clearing whole columns, it clears the digit from any cell that can see both of the far ends. That's usually two cells, sometimes more.

The same shape read the other way round is two columns sharing a row. Both directions are worth a scan, and the one you find first depends on which way you were reading the grid.

Example

Skyscraper: a worked example.

  1. Find two rows with two homes

    Follow one digit and look for two rows in which it can go in exactly two cells. That's the starting shape, and nothing else in the pattern matters until you have it.

  2. Look for the shared column

    Check whether one of the two cells in the first row sits in the same column as one of the two cells in the second. If the two rows share no column, keep looking; if they share both, you have an X-Wing and should use that instead.

  3. Read the two ends

    The cell that isn't in the shared column in each row is an end. The digit must occupy at least one of the two ends, because if both were empty each row would have only its base cell left and the shared column couldn't hold the digit twice.

  4. Clear what sees both ends

    Remove the digit from any cell that shares a unit with both ends. That always includes the two cells where each end's row crosses the other end's column, and sometimes a few cells inside the ends' boxes as well.

Spotting it

Skyscraper: how to spot it.

  • Two rows where a digit has exactly two homes are easy to spot and worth collecting: they feed X-Wings, swordfish and skyscrapers alike.
  • The shared column is the tell. Two such rows that overlap in one column and not in the other are a skyscraper, waiting for you to look at the far ends.
  • Remember the vertical reading. Two columns with two homes apiece sharing a row is the same pattern, and it's invisible from the rows.
  • If the rows share both columns it's an X-Wing, which removes more. Take the stronger elimination when both are available.

Where it stops working

A skyscraper places nothing and clears very little, so a correct pattern can still leave the grid stuck: it pays only when the two removals uncover a single candidate or a pair. It also needs the two homes in each row to be real, so an incomplete candidate list will invent a skyscraper out of a row that had three possible cells all along.

Keep going

Related topics.

New to sudoku? The rules take two minutes to read, and the first-puzzle walkthrough covers the basics that come before any of these.

Beyond the lessons.

The game's lessons stop at X-Wing, so there's no Skyscraper lesson. The eight it does teach will get you through almost any puzzle; this page is here for when you run into the pattern anyway.

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