Skyscraper

Also called Turbot Fish (row or column form), Sashimi X-Wing pair.

New to this shorthand? How to read a sudoku grid explains r4c7, houses, candidates and the rest.

What it means

Pick a digit. Find two rows where it has only two homes each. Two of those homes sit in the same column. The other two are the tops of the towers. One of those tops holds the digit, so any square that sees both tops cannot have it.

The exact rule, for stronger players

The rule, stated exactly: For digit d, two lines of the same orientation each containing d in exactly two cells, where one pair of those cells shares a line of the opposite orientation. The two remaining cells are the endpoints, and d is eliminated from cells seeing both.

What it removes: Remove d from cells that see both free-end cells.

Why it works: The shared line means at most one of the two joined cells holds d, so at least one of the two free ends does. Any cell seeing both free ends is therefore excluded.

Worked examples

These come from real Proven Sudoku boards. Nobody drew them by hand to make the pattern look neat. Each one is checked first, so the board is one you could really meet, the pattern is really there, and every digit it rules out is really wrong.

Sudoku grid showing Skyscraper. If r5c9 is not 4 then r5c3 is, which stops r6c3 being 4 and forces r6c7. So 4 sits in r5c9 or r6c7, and any cell seeing both can be cleared of 4. — If r5c9 is not 4 then r5c3 is, which stops r6c3 being 4 and forces r6c7. So 4 sits in r5c9 or r6c7, and any cell seeing both can be cleared of 4.

Sudoku grid showing Skyscraper. If r5c4 is not 7 then r1c4 is, which stops r1c6 being 7 and forces r6c6. So 7 sits in r5c4 or r6c6, and any cell seeing both can be cleared of 7. — If r5c4 is not 7 then r1c4 is, which stops r1c6 being 7 and forces r6c6. So 7 sits in r5c4 or r6c6, and any cell seeing both can be cleared of 7.

Learn these first

Turbot FishPick one digit.

Related single-digit patterns

X-ColoringColouring with an extra step.
2-String KitePick a digit.
3D MedusaColouring spread across every digit at once, not just one.
Empty RectangleIn one box, all the homes for a digit sit in one row and one column.
Multi-ColoringColouring marks a digit's spots in two shades.

How often does it come up?

Our rough estimate: the most common Turbot specialization and one of the more frequent intermediate patterns overall.

Play today's puzzle or read how difficulty is graded.