Skyscraper Sudoku Technique: How It Works on Real Puzzles

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Published April 13, 2026 · Updated October 14, 2026

Puzzle 205 of our Evil sudoku set has 27 digits at the start. Thirteen hidden singles, two naked singles and five pointing pairs later, 39 squares are still empty. Our Sudoku Helper tries all eighteen of its techniques here, and just two find anything: an Almost Locked Set, and a Skyscraper on the 6s. It takes two candidates off the board, both squares become naked singles, and from there naked singles alone finish the puzzle.

The Skyscraper rarely gets top billing among the advanced techniques, so we counted. Across the first 500 puzzles of our Evil bank, the helper needs one on 114, more often than it needs an X-Wing (74). On our Easy to Expert levels it never needs one. It is one digit and four squares, and once you know the only places its eliminations can land, it takes seconds to check.

What is a Skyscraper in sudoku?

A Skyscraper needs one digit and two columns, each with exactly two places left for it. If one place in the first column shares a row with one in the second, those two squares are the base, and the two that don’t line up are the roofs. One of the roofs must hold the digit, so any square that sees both roofs can’t.

The rotated version uses two rows that hold the digit twice each, with the base sharing a column. Our helper calls the loose ends roof cells, and its hint for puzzle 205 reads like a recipe: “Skyscraper on 6 — columns 3 and 7 share row 5. Remove 6 from cells seeing both roof cells.”

Why does a Skyscraper work?

Start at one roof and follow the 6s. Suppose R2C3 is not 6. Column 3 then has nowhere for its 6 but R5C3. Row 5 can hold only one 6, so R5C7 is not 6, and column 7 has to put its 6 in R1C7. Start from R1C7 instead and the same steps end at R2C3. Either way, at least one of R2C3 and R1C7 is a 6, maybe both, and a square that sees both of them cannot be.

Two squares on this board see both roofs and still have a 6: R1C2, which shares row 1 with R1C7 and box 1 with R2C3, and R2C9, which shares row 2 with R2C3 and box 3 with R1C7.

None of the earlier moves removed a 6, so the 6s drawn are exactly the ones the digits allow. This position is ready in our Sudoku Helper. Next Hint pencils in the candidates on its first press; the second offers a pointing pair and reports six techniques available. Choose Skyscraper from the Available Techniques menu to see this one, or apply the three pointing pairs the helper offers first, as our replay did, and the Skyscraper is the next hint.

Which squares can a Skyscraper clear?

Fewer than “any square that sees both roofs” makes it sound. On puzzle 205 each roof sits in a column that holds the 6 only at that roof and its base, so nothing else in that column has a 6 to lose. A square has to see each roof some other way: along the roof’s row, or inside its box. That leaves only the places where one roof’s row runs through the other roof’s box, three squares for each roof and six at most.

Those places exist only when the two roofs are in the same band of three rows. On puzzle 205 they are, in rows 1 and 2, so row 1 crosses box 1 and row 2 crosses box 3: the struck and shaded squares in the first figure. Had the roofs been in rows 2 and 8, the only squares seeing both would lie in the two link columns, where no 6 is left. A Skyscraper like that is genuine and removes nothing.

When the strong links run along rows, swap the words. The roofs must share a stack of three columns, and the eliminations fall where each roof’s column crosses the other roof’s box. Puzzle 119 has one, twelve moves in.

In the first 500 Evil puzzles the helper played 126 Skyscrapers. Every one had its roofs in one band or one stack, and each of the 228 candidates they removed sat at one of those crossings. Most removed two; none removed more than three. 84 had their strong links in rows and 42 in columns, so search both ways.

How do you spot a Skyscraper?

Hunting for the shape among a full set of pencil marks is slow. Turn each line into a short list of numbers, working one digit at a time:

1. List every row that has the digit in exactly two places, as pairs of columns. On puzzle 119 the 5s give row 4 → 1, 6 and row 7 → 3, 6.
2. Find two pairs that share exactly one number. Here it is 6, so the base is in column 6, and the other two numbers, 1 and 3, are the roofs’ columns. Two pairs that share both numbers are an X-Wing.
3. Check that the roofs’ columns fall in one stack: 1–3, 4–6 or 7–9. Columns 1 and 3 do. If they don’t, move on, because that Skyscraper removes nothing.
4. Look where each roof’s column crosses the other roof’s box. Any of those squares still holding the digit loses it.
5. Repeat with the columns, listing rows, then go to the next digit.

Start with digits that are partly placed. On 94 of the 126 Skyscrapers the digit was already on the board three to five times, and never six or more. And count from complete pencil marks: a row with a missing mark looks just like a row with two places, and a Skyscraper built on it can strike out the digit that belongs there.

Is a Skyscraper just an X-Wing with one corner moved?

Yes, and the band rule says how far the corner may go. On puzzle 205, if column 7’s upper 6 were in R2C7 instead of R1C7, the four squares would form a rectangle: an X-Wing on columns 3 and 7, rows 2 and 5. The Skyscraper is that X-Wing with one corner slid up its column. While the corner stays inside its box, as R1C7 does, the pattern can still remove something; once it leaves the box, it can’t.

Seen that way, a Skyscraper that removes anything is a sashimi X-Wing: a finned X-Wing with a corner missing and the fin in that corner’s box. The finned-fish detectors we tested alongside the helper look only for ordinary fins, so they never play a Skyscraper as such, but placed ahead of the wings they often get in first. With them and three other extra techniques, our solver needed a Skyscraper on only 44 of the 500 puzzles: at 69 positions where the helper would have played one, a finned X-Wing or a finned Swordfish came first. We quote the helper’s 114, because that is what its hints do.

A Skyscraper is also the shortest open X-chain: strong down column 3, weak along row 5, strong up column 7, with the digit going from every square that sees both ends. Some solvers call every four-square X-chain a Turbot Fish, Skyscrapers included. Our X-cycles guide covers the whole family, including the other four-square shapes: the 2-String Kite, whose two strong links run along a row and down a column with their near ends sharing a box, and the Turbot Fish proper, such as a chain with a strong link inside a box. Neither is among the helper’s eighteen, so when the helper stalls they are worth a look.

Simple Colouring builds its chains from strong links alone. Row 5 of puzzle 205 holds three 6s, so colouring cannot cross it, and at this position the helper finds no colouring move. That weak middle link is what a Skyscraper can use and colouring can’t.

How often do you need a Skyscraper?

The counts below come from letting the helper solve puzzles from our own levels one hint at a time, so the simplest of its eighteen techniques always moves first. A puzzle counts if a Skyscraper was ever that move.

• Our classic levels: never. Not one of the 800 puzzles we tried, 200 per level across Easy, Medium, Hard and Expert, needed it. Those levels deal only boards that singles finish, with locked candidates added on Hard and naked pairs on Expert. Of the 277 Expert positions where no single remained, 73 had a Skyscraper in them, and each of those also offered an easier move.
• Evil: nearly one puzzle in five. 114 of the first 500 needed one, and 908 of all 5,000.

Needed: puzzles where, at some point, the technique was the simplest move among the helper’s eighteen, tried in the helper’s order (X-Wing, the wings, Skyscraper, then Simple Colouring). Last hard step: puzzles the helper finished with nothing further down its list. Because the bank is kept in score order, the first 500 run harder than all 5,000 together. Measured October 2026.

Stuck after a Skyscraper, or can’t find one?

Look first at the squares that lost the digit, then at the roofs’ boxes. 57 of the 126 Skyscrapers in our count left a single straight away. On puzzle 205 both struck squares became naked singles, and R2C3 became the last place for a 6 in box 1.

When nothing opens, try an Almost Locked Set, the only technique below the Skyscraper on the helper’s list that these puzzles often need. Of the 114 puzzles that needed a Skyscraper, the helper finished 86: 36 with nothing further down its list, 47 that also needed an Almost Locked Set, and 3 that also needed Simple Colouring. The other 28 it could not finish.

If you can’t find a Skyscraper at all, it is most likely not there. Only 22.1% of Evil positions with no single left had one; Simple Colouring was there at 43.2% and an Almost Locked Set at 95.1%. Beyond the helper’s list, try the four-square X-chains above, then XY-chains; a BUG is worth checking for when only a few squares remain, and forcing chains are the fallback. Our guide to sudoku techniques sets out the full order.