SUDOKU VARIANT GUIDE
XV Sudoku: rules, strategy and how to start
X means the pair sums to ten, V means it sums to five. Two letters that quietly sort the whole board into low and high digits.
Why the rule changes everything
A V is one of only two pairs — 1-4 or 2-3 — so it is a very small door. Two Vs in the same row can pin a 1 immediately.
An X is wider but still eliminates: it forbids 1-1, 2-2 and pairs that sum to anything else, so a cell with a known 6 forces the neighbour to be 4.
How to start a board
- Resolve every V first — there are only four ways to place one and most die instantly.
- For X, list the legal partners of each digit: 1↔9, 2↔8, 3↔7, 4↔6, 5↔5. Note that 5-5 is legal unless the cells share a unit.
- Work from the edges of the grid inward; the border cells have fewer competing units.
Common mistakes
- Applying the rule to unmarked neighbours. Only X and V pairs are constrained here.
- Forgetting 5-5 is a valid X — it is the only pair that repeats.
- Treating X and V as decoration on the boxes rather than as pair constraints that cross boxes.
Frequently asked questions
Can a V pair be 3 and 2 in either order?
Yes, order is irrelevant; only the sum matters.
Does an X have to be adjacent horizontally?
In this board, X and V only appear between orthogonally adjacent cells.
Every board on this site is generated with a uniqueness check, so the puzzle in front of you has exactly one solution and every digit can be proved rather than guessed.
Play XV Sudoku ↗