Skyscraper
A digit in two lines sharing one base clears the roofs’ peers.
A Skyscraper is a single-digit pattern built from two "strong links": one candidate digit is pinned to exactly two cells in each of two rows (or two columns), and one cell from each pair lines up in the same column (or row) — the shared "base". Because a Skyscraper sudoku pattern forces the digit onto one of the two mismatched "roof" ends, any cell that can see both roof ends is barred from holding that digit, letting you eliminate it.
Worked example
Here digit 1 is confined to just two cells of row 1 — R1C1 and R1C4 — and two cells of row 2 — R2C1 and R2C6. The base cells R1C1 and R2C1 both sit in column 1, and since a column can hold only one 1, they cannot both be the answer; that forces the digit onto one of the offset roof ends, R1C4 or R2C6. R3C5 lies in box 2 alongside both roof ends, so it sees whichever end turns out to be a 1 — which means 1 can be removed from R3C5.
When to use it
Reach for a Skyscraper once singles, locked candidates and naked/hidden pairs have stalled and you need a single-digit break-in before trying anything chain-heavy. Scan digit by digit for one that has exactly two candidate cells in two separate rows (or two separate columns) where one cell of each pair shares the same column (or row): those aligned cells are the base, the two offset cells are the roofs. Then check the box and lines around the roofs for any cell that sees both — that cell is your elimination.
How to use it
- Find two strong lines. Find a digit that appears exactly twice in each of two rows (or two columns).
- Match the base. The two lines must share one column (the base); their other two cells are the roof, in different columns.
- Eliminate from the roofs’ peers. At least one roof holds the digit, so remove it from every cell that sees both roof cells.
Frequently asked questions
What is a Skyscraper in Sudoku?
A Skyscraper is a single-digit technique using two strong links. Pick a digit that appears in only two cells in each of two rows; if one cell from each row sits in the same column (the base), the digit must end up in one of the two remaining cells (the roofs). Any cell that sees both roofs — in the example above, R3C5 seeing R1C4 and R2C6 through box 2 — cannot contain that digit, so you erase it there.
How do I spot a Skyscraper on the grid?
Work one digit at a time. Find two rows where the digit has exactly two candidate spots each, then check whether one spot from each row falls in the same column — that alignment is the base. The two non-aligned spots are the roofs. Confirm the pattern is useful by looking for a cell (or cells) that can see both roofs at once; if none exists, the Skyscraper eliminates nothing. Columns work the same way with the roles of row and column swapped.
What is the difference between a Skyscraper and an X-Wing?
Both use a digit locked to two cells in two rows (or columns), but an X-Wing needs all four cells to share the same two columns — a perfect rectangle — and it clears the digit down those columns. A Skyscraper is the near-miss: only one end lines up (the base) while the other end is offset (the roofs), so instead of clearing whole lines it removes the digit from cells that see both offset roof ends, such as R3C5 in the worked example.