❎ How to play XV Sudoku

By Miguel Arechaga

XV Sudoku is an elegant sum-based variant that borrows from Roman numerals: X is worth 10 and V is worth 5. These letters appear between some cells and tell you what the two cells they separate add up to. It's simple to understand yet very powerful for deduction. Here are the rules and a few tips.

What is XV Sudoku?

It's a standard 9x9 sudoku where, between some neighboring cells, an X or a V appears. An X means the two cells sum to 10; a V means they sum to 5. In the full version, the absence of a mark also carries information: if two neighbors have neither an X nor a V, their sum is not 5 or 10.

Rules and how to play step by step

1. The sudoku rules apply: 1 to 9 without repeats in each row, column and 3x3 box.
2. X: the two cells sum to 10 (1+9, 2+8, 3+7, 4+6).
3. V: the two cells sum to 5 (1+4, 2+3).
4. No mark: the sum is neither 5 nor 10. Every puzzle has a unique solution.

Tips to solve XV Sudoku

• V marks are very restrictive: there are only two ways to sum to 5 (1+4 and 2+3), so a V narrows the options a lot.
• X marks have four pairs: 1+9, 2+8, 3+7 and 4+6; combine with the box and column to narrow it down.
• The 5 never touches a mark: it can't sum to 5 (it would need a 0) or 10 (it would need another 5, forbidden on the same line), so a 5 is never next to an X or a V.
• Don't forget the absence of a mark: it rules out two neighbors summing to 5 or 10. Combine it with the classic techniques.

How it differs from regular sudoku

Compared to classic sudoku, XV adds sum relationships (5 and 10) between neighboring cells. It's a cousin of Kropki Sudoku, which uses dots instead of letters. If you enjoy sum-based variants, you'll also enjoy Killer Sudoku.