Skyscraper
The Skyscraper is one of the friendliest single-digit techniques — often easier to spot than an X-Wing. It works with just one digit and four cells, using the fact that a digit with only two homes in a line must be in one of them.
What is a Skyscraper?
Take a single digit — say 6. Find two rows in which the 6 can go in only two cells each, and where one of those cells shares a column across the two rows. Those aligned cells are the "base"; the two other cells are the "roof". Here's the logic: the two base cells can't both be 6 (same column), so at least one roof cell is a 6. Any cell that can see both roof cells therefore can't be 6.
The 6 sits in two cells in rows 1 and 5, both using column 1 (blue base). The far ends (amber roof) at r1c6 and r5c8 mean any cell seeing both — the red cells — can't be 6.
Step by step
- Pick a digit and find two lines where it has exactly two homes each.
- Check for a shared cross-line. One home in each line must sit in the same column (or row) — that's the base.
- Find the roof. The other two homes are the far ends.
- Eliminate the digit from any cell that sees both roof cells.
Where it fits
The Skyscraper belongs to the Turbot Fish family of two-strong-link patterns, alongside the 2-String Kite and Empty Rectangle. They all share the same core idea — two conjugate pairs, one elimination — just wired differently.
Common mistakes
- No shared line. If neither end lines up in a column or row, it isn't a skyscraper.
- Eliminating at the base. The elimination is at cells seeing both roof ends, not the aligned base.
Which Sudoku types is this best for?
The same logic applies across variants, but it pays off more in some than others.