How to Solve Sudoku: Skyscraper

2 rows where a digit has only 2 places. If one end of each lines up in the same column, the digit can be removed from any cell that sees both of the 2 ends that do not line up.

On Puzzle Lounge it appears at Master.

See it step by step

  1. A board full of notes. Look only at the 9s (the small digits are the notes, or candidates)
  2. ● = a cell where 9 can go (notes other than 9 have been removed)
  3. In row 3, 9 has only 2 places
  4. Row 8 also has only 2 places (one end of each lines up in column 9)
  5. 9 can only go in these 3 ways (it cannot be in both of the 2 lined-up cells)
  6. In every case, one of the remaining ends is 9 (row 3, column 2 or row 8, column 1)
  7. So a 9 that sees both ends can be removed (9 goes from row 2, column 1)

This is Master No.3, partly solved.

Solve this puzzle →

How to find it

  1. Pick a digit and list the rows (or columns) where it has only 2 candidates.
  2. Among them, find 2 where one end of each lines up in the same column (or row).
  3. See whether any cell that sees both of the 2 ends that do not line up has that digit as a candidate.

Things to watch

  • If both ends line up, that is an X-Wing.
  • Each of the 2 must have exactly 2 candidates.
Read the full explanation

When 2 rows (or columns) each have only 2 candidates for a digit and one end of each lines up in the same column (or row), the digit must be in one of the remaining 2 ends. So it can be removed from any cell that sees both of those 2 ends.

It looks like an X-Wing, but one side of the 4 corners is out of line. Suppose a digit (say 9) can go in only 2 places in both row A and row B, and one place from each is in the same column X.

Column X's 2 cells cannot both be 9. If row A's 9 is not in column X, the other end of row A is 9. The same goes for row B. So one of the 2 ends that do not line up must be 9.

So 9 cannot go in any cell that sees both of those 2 ends. The name comes from the shape, like 2 tall buildings side by side.

9 can go only in the 2 places row 3, column 9 and row 3, column 2, and the 2 places row 8, column 9 and row 8, column 1. Row 3, column 9 and row 8, column 9 are on the same line, so they cannot both be 9. So one of row 3, column 2 and row 8, column 1 is 9, and 9 can be removed from the candidates at row 2, column 1, which sees both.

Other techniques

Hidden SingleNaked SinglePointingClaimingHidden PairHidden TripleCage Combinations (Killer)Rule of 45 (Killer)Naked PairNaked TripleX-WingXY-WingSwordfish2-String KiteXYZ-WingW-WingJellyfish

For the basics and how to use notes, see how to solve sudoku, with tips. If you get stuck, the game's hints guide you step by step, starting with where to look.

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