Skyscraper
A Skyscraper uses two parallel strong links — two rows or two columns where a digit is confined to exactly two cells. One cell from each pair shares a perpendicular line (the bridge), so they can't both be the digit. That forces at least one of the other two endpoints (the roof) to contain the digit, eliminating it from any cell that sees both roof cells.
What is a Skyscraper?
A Skyscraper is a single-digit pattern built from two strong links running in parallel — two rows, or two columns, where a particular candidate appears in exactly two cells. The four cells form two pairs, and one cell from each pair lies on the same perpendicular line. Those two shared-line cells are the bridge: because they see each other, they can't both hold the digit. Since each strong link must place the digit in one of its two cells, and the bridge can't supply both, at least one of the other two endpoints — the roof — has to contain the digit. Any cell outside the pattern that shares a house with both roof cells can't also hold the digit, so it gets eliminated.
How to spot it
Skyscrapers don't jump out the way singles or locked candidates do — you find them by scanning one digit at a time across parallel houses. Pick a candidate digit and look for rows (or columns) where it's restricted to exactly two cells. When you find two such rows, check whether one cell from each pair shares a column. If they do, you've got a Skyscraper: the shared-column cells are the bridge, and the other two are the roof. The elimination lands on any cell that sees both roof cells — same row, same column, or same block as each. Skyscrapers turn up fairly often in harder puzzles once the easier techniques have been exhausted, and they're worth checking whenever you see two strong links on the same digit in parallel houses.
The logic
Each strong link is a house where the digit has exactly two candidate cells, so one of them must be the digit. In a Skyscraper, one candidate from each link sits on a shared perpendicular line — the bridge. Because the two bridge cells see each other, at most one of them can be the digit. If the first bridge cell is the digit, the second link's digit falls on its roof cell. If the second bridge cell is the digit, the first link's digit falls on its roof cell. And if neither bridge cell is the digit, both roof cells must be. In every case, at least one roof cell contains the digit, so any cell that sees both roof cells can't also hold it. The same reasoning applies with rows and columns swapped.