Sudoku technique · First needed at: Extreme ●●●●●●●●

Two-string kite

The number has two places in a row and two in a column, and one of each shares a box — so anything seeing both far ends is out.

An example from a real puzzle

How to spot it

1. Find a number with exactly two places in a row and exactly two in a column.
2. One place from each sits in the same box.
3. A cell that sees both of the far ends cannot hold the number.

Why it works

The two ends inside the box cannot both hold the number, so at least one far end does — and a cell seeing both far ends loses it.