Skyscraper
Advanced Removes candidates Used in: Sudoku, Killer Sudoku
Two rows each have a digit confined to exactly two columns, and they share one of those columns (the “base”). The digit must lie in one of the two far ends (the “roofs”), so it is removed from any cell that sees both roofs.
How to spot it
Two columns (or rows) where a digit forms a conjugate pair, sharing the same base line.
What you can conclude
Eliminate the digit from any cell that sees both roof ends.
Worked example
Two columns hold 5 as a pair, sharing a base row. One of the far roofs must be 5 — so 5 leaves any cell seeing both roofs.