Skyscraper Sudoku Strategy: A Clear Candidate Guide
A Skyscraper uses two strong links for one candidate digit. In two rows, that digit appears exactly twice. One endpoint from each row shares a column, while the other two endpoints form the roofs. At least one roof must be true, so a cell that sees both roofs cannot contain the digit.
Align one end of two strong links; the unaligned ends become the roofs that drive the elimination.
What is a Skyscraper?
A row-based Skyscraper begins with two rows containing exactly two candidates for digit X. One candidate in each row lies in the same column. The other endpoints lie in different columns. Because each row must place X, at least one outer endpoint is X. The column-based form swaps rows and columns.
How to find a Skyscraper
- Choose one digit. Look for rows where that candidate appears exactly twice.
- Pair two strong links. The rows should have one endpoint aligned in the same column.
- Identify the roofs. The two unaligned endpoints are the cells that force the conclusion.
- Find a shared peer. Remove the digit from a cell that sees both roofs.
Example: a Skyscraper on candidate 6
Row 2 places 6 in c3 or c8. Row 7 places 6 in c3 or c5. The c3 endpoints align, leaving r2c8 and r7c5 as the roofs.
- Confirm both links. Each selected row has exactly two candidates for 6.
- Use the aligned column. If one c3 endpoint is false, its row forces the corresponding roof.
- Eliminate at the roof intersection. Any cell that sees r2c8 and r7c5 cannot contain 6.
Common Skyscraper mistakes
Using weak row links
Each base row must contain exactly two candidates for the target digit.
Eliminating from the tower
The target is a common peer of the two roofs, not of the aligned endpoints.
Mixing candidate digits
All four endpoints and the elimination must concern the same digit.
Skyscraper practice checklist
- Mark conjugate pairs for one digit in rows or columns.
- Look for two links sharing one cover house.
- Trace visibility from both outer endpoints before eliminating.
Skyscrapers become easier once strong links are visible. Confirm exactly two candidates in each base house, then let the two roofs define the only valid elimination area.
Skyscraper FAQ
Is a Skyscraper a fish pattern?
It is closely related to single-digit fish and chain logic, but it uses two strong links with one aligned endpoint pair.
Can a Skyscraper be column-based?
Yes. Use two columns with exactly two candidates and align one endpoint pair in a row.
What can be eliminated?
Only the target digit from cells that see both unaligned roof endpoints.
