Skyscraper
A single-digit chain: two rows each have the candidate in exactly two cells, sharing one column. The far ends eliminate the candidate from cells that see both.
Candidate 4 forms two conjugate pairs — one in this row, one in this row — sharing a column.
How to spot it
- 1Find two conjugate pairs of a digit in two rows sharing a column (the 'base').
- 2The two off-column ends form the 'roof'.
- 3Remove the digit from any cell that sees both roof ends.
When to use
A friendlier alternative to the X-Wing — often easier to spot.
When not to
If the base cells don't share a line, it's not a skyscraper.
Common mistake
Confusing the aligned base with the eliminating roof ends.
Worked Example
A skyscraper starts with two conjugate pairs of the same digit — say two rows where a candidate appears in exactly two cells each — that share one column between them (the "base"). The two remaining, unshared ends form the "roof."
Since each row must place its candidate at one end of its pair, the roof ends effectively behave like a strong link themselves. Any cell that sees both roof ends can't hold that candidate, since one roof end or the other is guaranteed to. It's often easier to spot than a full X-Wing rectangle, since you're only tracking two aligned pairs rather than four matching cells.