Skyscraper

Updated 15 October 2026

A Skyscraper is a single-digit pattern made of two strong links in parallel rows, or in parallel columns, whose bottom ends share a line: one of the two top ends, the roofs, must hold the digit, so any cell that sees both roofs cannot.

How Suuudokuuu finds a Skyscraper

Suuudokuuu’s engine has no separate Skyscraper detector, and this page will not pretend it does. A Skyscraper is the shortest possible X-Chain: strong link, weak link, strong link, four cells on one digit. The X-Chain detector searches shortest chains first, so a Skyscraper, at four cells, is among the first chains it can report, and the hint and the step-by-step solver label it X-Chain. The name is worth learning anyway, because the shape is far easier to spot by eye than a general chain.

The logic is the X-Chain’s. In each of the two rows the digit has exactly two cells, so if one is false the other is true — that is a strong link. The two base cells share a column, so they cannot both hold the digit — that is a weak link. Follow it from either roof: if the first roof is not the digit, its partner in the row is, which rules out the base cell below it, which forces the second roof. Either way, one roof holds the digit.

Worked example

A position from a Hard puzzle in the rated sample. On 9, row 3 and row 4 each have two candidates, and r3c1 and r4c1 share column 1. The roofs r3c3 and r4c2 cannot both be empty of 9, so 9 is erased from r5c3 and r6c3.
	c1	c2	c3	c4	c5	c6	c7	c8	c9
r1	3458	2	348	567	57	9	35678	1	45678
r2	35	36	13	8	157	4	23567	2567	9
r3	4589	7	1489	156	2	3	568	568	4568
r4	789	89	5	3	6	28	278	4	1278
r5	34789	1	234789	249	489	5	2678	26789	2678
r6	6	489	2489	1249	1489	7	258	3	1258
r7	2	389	6	579	35789	1	4	578	578
r8	1	48	478	2457	4578	28	9	25678	3
r9	34789	5	34789	2479	34789	6	1	278	278

Pattern cells: r3c3, r3c1, r4c1, r4c2
Eliminations: 9 from r5c3, 9 from r6c3

Row 3 holds 9 only in r3c1 and r3c3, and row 4 only in r4c1 and r4c2. Those are the two floors of the Skyscraper. The base cells r3c1 and r4c1 stand in the same column, so at most one of them is 9, which means at least one of the roofs, r3c3 or r4c2, is 9. r5c3 and r6c3 sit in column 3 with r3c3 and in box 4 with r4c2, so they see both roofs and lose 9.

How to spot a Skyscraper

Pick a digit and find two-candidate linesFor one digit, look for rows, or columns, where it has exactly two candidates. Each is a strong link.
Line up the basesFind two such rows where one candidate from each sits in the same column. Those two cells are the base of the Skyscraper.
Mark the roofsThe remaining candidate in each row is a roof. The roofs must not share a column, or the pattern is an X-Wing instead.
Erase the digit where both roofs are seenRemove the digit from every cell that sees both roofs, through a row, a column or a box.

Common mistakes

Using a row with three candidates for the digit. The pattern needs strong links, so each line must hold exactly two.
Lining the roofs up as well as the bases. Four candidates in two columns is an X-Wing, which eliminates along the columns instead.
Erasing the digit from a base cell. The bases are part of the chain; only cells that see both roofs are cleared.

The Skyscraper has two close relatives: the 2-String Kite joins a row link to a column link through a box, and the Turbot Fish is the general four-cell chain that both are special cases of. Every pattern the engine implements is listed on the technique index.

Skyscraper FAQ

Is a Skyscraper an X-Chain?

Yes. It is the four-cell X-Chain where both strong links run along parallel rows or columns, which is why Suuudokuuu’s engine reports it as an X-Chain.

What is the difference between a Skyscraper and an X-Wing?

In an X-Wing both pairs of candidates line up, forming a rectangle. In a Skyscraper only the bases line up and the roofs are offset, so the elimination falls on the cells that see both roofs.