The Skyscraper technique is usually the next sudoku tool worth reaching for once Naked and Hidden Singles, pairs, triples, and locked candidates are exhausted. It's the simplest member of a family of "chain" techniques — you're about to meet its two closest relatives, the 2-String Kite and the Empty Rectangle, right after this one.
What is a Skyscraper?
A Skyscraper uses two rows (or two columns) where a digit fits in exactly two cells each. One cell from each row lines up in the same column — those two form the "base" and can't both hold the digit. The other two cells, one per row, are the "roofs."
If the base cell in the first row is the digit, the base cell in the second row isn't, forcing the digit into the second row's roof cell. If it isn't, the first row's roof cell must be it. Either way, one of the two roof cells holds the digit — so any cell that sees both roof cells can't.
Why it works
The two base cells share a column, so they can't both be the digit. That leaves exactly two possibilities: base-row-1 is the digit (pushing the digit to row 2's roof), or base-row-2 is the digit (pushing it to row 1's roof). There's no third outcome, so one roof is always the digit — even though you don't know which.
A worked example
Here's the exact board the app uses for its own Skyscraper lesson — a real puzzle, not a schematic:
Work it with Naked and Hidden Singles, then Pointing Pairs — that clears most of the grid on its own:
That's as far as the easy tools take it. Digit 6 is the sticking point: row 2 has a 6 in only two cells, row 9 has a 6 in only two cells, and one cell from each lines up in column 7 — the base. That makes the other two, R2C5 and R9C4, the "either/or" roofs:
One of the two roof cells has to be the 6. Any cell that sees both roofs loses its 6 — here, that's R3C4 (shares a box with R2C5, shares column 4 with R9C4) and R7C5 (shares column 5 with R2C5, shares a box with R9C4). Clear those two 6s and singles finish the rest of the grid.
Watch the 60-second lesson
The same board, step by step, as the app teaches it.
How to spot one
- Pick a digit and find two rows (or two columns) where it fits in exactly two cells.
- Check whether one cell from each lines up in the same column (or row) — but the other two don't.
- That shared line is the base; the other two cells are the roofs.
- Eliminate the digit from any cell that sees both roofs — usually in the box they share, or the rows/columns between them.
Skyscraper vs X-Wing
The Skyscraper is often called a "broken X-Wing," and comparing the two is the quickest way to understand both. They start the same way: one digit, two rows (or two columns), and exactly two candidate cells in each.
| X-Wing | Skyscraper | |
|---|---|---|
| Cells that line up | Both pairs, in the same two columns | Only one pair (the base) |
| Shape | A perfect rectangle | A rectangle with one corner knocked out of line |
| What you eliminate | The digit from the rest of both columns | The digit from cells that see both roofs |
| How often it appears | Less often — needs an exact match | More often — needs just one shared column |
In practice you find them with the same scan. When you're looking for an X-Wing and two rows match on one column but not the other, don't move on — you've found a Skyscraper. Its eliminations are smaller than an X-Wing's, but they're often exactly the one or two cells you need to get going again.
Common mistakes
- Three or more candidate cells don't count. Each of the two base lines needs exactly two.
- Only one pair of cells may share a line. If both pairs share a column, that's an X-Wing, not a Skyscraper.
- You eliminate from cells that see both roofs — not the base cells, and not cells that only see one roof.
Practice this on a real board
Sudoku Mentoring names the technique on your board, highlights the exact cells, and gives you a guided practice puzzle. Free to start, no ads, offline.