What is a Skyscraper?
A Skyscraper is one number with exactly two possible cells in each of two rows, where one cell of each row shares the same column (the base) and the other two cells (the tops) do not. At most one base cell can hold the number, so at least one top does. Any cell that sees both tops can’t hold it.
It is an X-Wing with one corner bent out of line, so it is often the next fish-like pattern people learn. Columns can play the role of rows too. In Marble Sudoku you look for it one marble color at a time.
Use one in 4 moves
- 1Pick. One number at a time.
- 2Rows. Find two rows where it has exactly two spots.
- 3Base. Check that one spot of each row shares a column. The other two are the tops.
- 4Clear. Remove the number from every cell that sees both tops.
Then look for singles where you removed it.
TRY IT · GUIDED EXAMPLE
Build the Skyscraper
Only Pink is shown in color. Faded Pink marbles mark every hole where Pink could still go. Find Pink’s two spots in each glowing row, spot the shared base, and clear what sees both tops.
This is the real board from Marble Sudoku. The nine marble colors stand for the numbers 1 to 9.
- Rows
- Tops
- Clear
- Place
Build the Skyscraper
Read the example in words
Row 1 can only take Pink in columns 1 and 8, and row 7 only in columns 1 and 7. Both rows share column 1, the base. Column 1 holds only one Pink, so at least one row puts its Pink on its top: row 1, column 8 or row 7, column 7. Row 3, column 7 sees both tops (the same box as one, the same column as the other), and so does row 8, column 8. Both lose Pink, and row 3, column 7 is left with a single color: Sky. With numbers turned on, Pink is 3 and Sky is 9.
YOUR TURN · PRACTICE
Spot the Skyscrapers
Each round shows the spots for one color. Tap the four spots of a Skyscraper: two rows or two columns with exactly two spots each and one shared base line. On Easy the lines glow. On Hard you find them yourself.
Round color:
Why a Skyscraper works
Row 1 needs its 3 in one of two cells, and so does row 7. The two base cells sit in the same column, and a column holds only one 3. So at most one base cell is a 3, and at least one row has to place its 3 on its top cell instead.
We don’t know which top it is, and we don’t need to. A cell that shares a row, column or box with both tops would clash with either one, so it can’t be a 3. If both tops turn out to be 3, that is fine too: the logic only says “at least one”.
How to spot Skyscrapers faster
- Start from two-spot lines. Pick one number and mark every row (or column) where it has exactly two spots. Skyscrapers are built only from those.
- Look for a shared base. Two of those rows need one spot each in the same column. If both spots match, it’s an X-Wing instead.
- Check the tops’ reach. The tops are usually in the same band of boxes. The cells that see both are often in the boxes next to them.
- Turn the board. Do the same with columns: two columns with two spots each, sharing one row as the base.
Skyscraper vs. X-Wing vs. 2-String Kite
| Skyscraper | X-Wing | 2-String Kite | |
|---|---|---|---|
| Lines used | Two rows (or two columns) | Two rows (or two columns) | One row and one column |
| Shape | One end lined up, the other bent | Both ends lined up: a rectangle | Two ends meet in one box |
| Removes | From cells that see both tops | From the rest of both columns | From the cell that sees both free ends |
| Difficulty | Advanced | Advanced | Advanced |
Common mistakes
- Lines with three spots. Each of the two rows must have exactly two spots for the number. A third spot breaks the logic.
- Clearing the base. The base cells keep the number. Only cells that see both tops lose it.
- Cells that see only one top. A cell must see both tops. If it sees only one, the other top could be the true one.
- Tops in one line. If both tops are in the same column, it’s really an X-Wing. If they see each other, there is nothing to remove.
Skyscraper FAQ
What is a Skyscraper in Sudoku?
A Skyscraper is a number with exactly two spots in each of two rows (or columns), where one spot of each lines up in the same column (or row). The other two spots are the tops, and the number can be removed from every cell that sees both tops.
Why is it called a Skyscraper?
Draw the two rows and the base column and you get two towers of different heights standing on the same base line. The tops of the towers are the cells that matter.
Is a Skyscraper a kind of X-Wing?
It is a close relative. In an X-Wing both pairs of spots line up, so the pattern is a rectangle. In a Skyscraper only one end lines up. Both are single-number patterns, and both are sometimes called turbot fish.
Does it work with columns?
Yes. Two columns with exactly two spots each, where one spot of each shares a row, work the same way. Then you clear the cells that see both tops.
Does this work in Marble Sudoku?
Yes. Pick one marble color, find two rows with exactly two holes for it, and check whether one end of each lines up. Remove the color from every hole that sees both of the other ends.
When should I look for one?
After singles, pairs, box/line reductions and X-Wings stop finding moves. Skyscrapers are common in hard and expert puzzles.
PRACTICE IN THE GAME
Find Skyscrapers in a real puzzle
When a hard puzzle stalls, pick one color and look for rows with only two holes for it. Two of them standing on one base often break the puzzle open.
Next techniqueY-Wing
