Sudoku technique · First needed at: Expert ●●●●●○○○

Skyscraper

Two lines hold the number in only two places each and share one of them — so anything seeing both far ends is ruled out.

An example from a real puzzle

How to spot it

1. Find a number with exactly two places in each of two rows.
2. One end of each lines up in the same column — the base. The other two ends do not — the tops.
3. Remove the number from cells that see both tops.

Why it works

At most one base cell can hold the number, so at least one top must — and a cell that sees both tops loses it either way.