Skyscraper
Two rows hold a digit in two cells each, sharing one column.
Skyscraper is an easier-to-find variant on the X-Wing theme. Take two rows where a particular digit has exactly two candidate cells. If one column is shared between those rows (the "base"), the two unshared cells become the "roof."
One of the two roof cells must hold the digit — there's no other way for both rows to receive it without the base column carrying both. Anything that sees both roofs (shares a row, column, or box with each) is therefore unable to hold the digit.
Skyscrapers fire more often than X-Wings and reach cells the X-Wing pattern doesn't touch.
When the move applies
Sweep digit by digit, listing the rows that have exactly two homes for it. Any two such rows that share a column form a skyscraper.
The procedure
- Pick a digit.
- Find two rows (or two columns) where the digit has exactly two candidates each.
- Note the shared column (or row) — the base.
- The two off-base cells are the roof. Eliminate the digit from cells that see both roofs.