The Skyscraper Technique in Sudoku

The shape of a Skyscraper

Take one digit and find two rows where it has exactly two candidate cells each. If one end of both pairs lines up in the same column, that shared column is the base. The other two cells, sitting in different columns, are the tops of the two towers. The pattern is named for that image, two towers rising from a shared base.

Why it works

The base column can only hold the digit in one of its two cells. Whichever base cell is false forces its own row to place the digit in the top cell instead. So at least one of the two tops must be the digit. That means any cell seeing both tops cannot be the digit, because one of the tops has already claimed it.

How to use it

1. Choose a digit and mark the rows where it has only two candidate cells.
2. Find two such rows that share a column at one end.
3. Identify the two top cells, the ends that are not in the shared column.
4. Erase the digit from every cell that sees both tops.
5. Try the same search using columns instead of rows.

A close cousin

The Skyscraper is closely related to the Two-String Kite and the X-Wing. All three chain two conjugate pairs of a single digit and remove the digit from cells that see both loose ends. Learn the Skyscraper first, since its shape is the clearest, and the others will feel familiar.