Skyscraper

A digit in two lines sharing one base clears the roofs’ peers.

Learn the Skyscraper Sudoku technique: a digit in two rows (or cols) sharing one base column forces a roof, eliminating it elsewhere.

How to use it

1. Find two strong lines. Find a digit that appears exactly twice in each of two rows (or two columns).
2. Match the base. The two lines must share one column (the base); their other two cells are the roof, in different columns.
3. Eliminate from the roofs’ peers. At least one roof holds the digit, so remove it from every cell that sees both roof cells.