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.
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The Skyscraper guide on Sudoku247Wiki walks through the logic step by step, with worked examples, diagrams and FAQs.
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