Skyscraper
Two rows each with exactly two candidate cells for a digit, where two of the four endpoints share a column. The other two endpoints' shared-house cells lose the digit.
Advanced sudoku technique
What it is
A Skyscraper is the simplest Turbot Fish. Pick a digit. Find two rows in which that digit has exactly two candidate cells each. Two of the four endpoint cells share a column — those are the 'base' of the skyscraper. The remaining two endpoints — the 'tops' — define the constraint: at least ONE of them must hold the digit. Why? Because in the two base cells (same column), the digit can appear at most once per column. If both base cells lacked the digit, both rows would force their tops to hold the digit instead, so each row's top contains the digit. If one base cell holds the digit, only one row's top is the alternative. Either way, at least one of the two tops holds the digit. Therefore any cell that sees BOTH tops drops the digit. The column-rooted version (two columns + a shared row) is symmetric.
When to use it
On expert puzzles when Swordfish and X-Wing miss. Spot it by finding two row strong-links that share an endpoint column.
Worked example
Digit 6 in row 2 fits only R2C3 or R2C8. In row 7 it fits only R7C3 or R7C5. R2C3 and R7C3 are the base (shared column 3); R2C8 and R7C5 are the tops. One of the tops must hold 6. R7C8 sees R2C8 via column 8 and R7C5 via row 7 — so R7C8 drops 6 from its candidates.
Try it
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