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Two-string kite
A row and a column tie into the same box, with two ends hanging out.
Structurally a cousin of skyscraper, the two-string kite swaps one row for a column. A digit has exactly two candidates in some row and exactly two in some column. One cell from each of those lines lives inside the same box — that's the anchor. The other two cells are the ends of the kite.
One of the two ends must hold the digit. Cells seeing both ends cannot.
Kites cover most of the patterns older sudoku books grouped under "turbot fish."
When the move applies
When digit-by-digit analysis has run dry, look for a digit with a two-cell home in some row and a two-cell home in some column where the row's cell and the column's cell sit in the same box.
The procedure
- Pick a digit.
- Find a row and a column that each hold the digit in exactly two cells.
- Check that one row-cell and one column-cell share a box (the anchor).
- Remove the digit from cells that see the two unshared ends.