# Crossover

**Crossover** is a basic technique for Sudoku-X, which says that if there are exactly two cells **A** and **B** with candidate **X** along a diagonal, then **X** can be eliminated from the other two cells that are seen by both **A** and **B**. Crossover can also be applied when there are exactly three cells with the candidate **X** along the diagonal, as long as one of the cells is **r5c5**, and the other two cells **A** and **B** are located at the same distance away from **r5c5**.

In other words, the crossover rule is simply Common Peer Elimination applied to the diagonals of the Sudoku-X. When there are exactly two cells along the diagonal having the candidate **X**, SudoCue replicates crossover using Simple Colors, i.e., a X-Chain containing two cells.

## Example

The following example illustrates the crossover technique.

One of the diagonals has exactly two cells having **1** as a candidate: **r3c7** and **r6c4**. Both of these cells see **r6c7**. Hence, **1** can be eliminated from **r6c7**.