Peak Sudoku

Skyscraper

Two rows (or columns) where a digit has exactly two positions, sharing one column (or row).
All techniques

Overview

Tier 4
Fish, wings, chains & uniqueness
Typical difficulty
Peak Score 60–75
Also known as
Turbot Fish Type 1, Single-Digit Chain

A Skyscraper occurs when two rows have a target candidate in exactly two cells each, and two of those cells share a column (the base). At least one of the other two cells (the roofs) must hold that digit.

The Skyscraper is a single-digit pattern from the Turbot Fish family. Think of it as an X-Wing that got knocked askew: in two rows, a target digit appears exactly twice. One end of each row lines up in the same column (forming the shared 'base'), while the other two ends ('roofs') do not line up. Because the base column can only contain one instance of the digit, at least one of the two roof cells must contain it. Any cell that sees both roof cells cannot contain that candidate.

Worked examples

1 / 5
Reading the exampleCells forming the patternCells losing candidatesEliminated candidates
In columns 6 and 9, the digit 3 can only go in two cells each, and both columns have one in row 4. At most one of those row 4 cells is 3, so R7C6 or R9C9 must be 3, and 3 can be removed from any cell that sees both.
Removes: 3 from R9C5, R7C8

How to apply it

Spotting the pattern and making the deduction, in order.

  1. Pick a digit and find two parallel rows where that digit appears in exactly two cells each.
  2. Notice that one end of each row shares the same column (the base), while the opposite ends sit in different columns (the roofs).
  3. Because the base column can only hold the digit once, at least one of the base cells must be false.
  4. If a base cell is false, the opposite roof cell in that row is forced to be true.
  5. Therefore, at least one of the two roof cells is guaranteed to hold the digit.
  6. Eliminate that candidate from any cell that has a direct line of sight to both roof cells.

Common mistakes

  • Allowing extra candidates in the base rows.
  • Eliminating candidates that only see one of the roof cells.
  • Forgetting that Skyscraper can also operate vertically (two columns with a shared row base).

Frequently asked questions

How is a Skyscraper different from an X-Wing?
In an X-Wing, both ends line up in the same two columns. In a Skyscraper, only one end lines up; the other two ends are offset in different columns.
Peak Score 60–752-String KiteOne strong link on a row and one on a column, sharing a cell in the same box.Peak Score 55–70X-WingA digit restricted to exactly two cells in two different rows, forming a rectangle.Peak Score 65–80W-WingTwo bivalue cells with identical candidates, bridged by a strong link on one digit.

Put Skyscraper into practice

Try a puzzle from the Tough tier (Peak Score 61–70).

Play a Tough puzzleAll 25 techniques