X-Sudoku (Diagonal Sudoku) follows all Classic rules and adds one extra constraint on the two main diagonals.

How is X-Sudoku different from Classic?

In addition to every row, column and box, each of the two long diagonals must also contain the digits 1 to 9 exactly once.

What do the marked diagonals mean?

The two highlighted diagonals are extra regions you must complete with 1–9, just like a row or column.

Is X-Sudoku harder than Classic?

The extra diagonals add constraints that give you more ways to make deductions, so many players find them a fun, slightly different challenge rather than strictly harder.

What are the diagonal rules in Sudoku?

Only the two main corner-to-corner diagonals are constrained, and each must contain the digits 1 to 9 exactly once — the same rule a row or column follows. Broken diagonals and off-centre diagonals carry no rule. A cell on a diagonal therefore belongs to four units, and the centre cell, which sits on both diagonals, belongs to five.

Is X-Sudoku the same as Sudoku X or Diagonal Sudoku?

Yes. X-Sudoku, Sudoku X, Diagonal Sudoku and X-Factor Sudoku are all names for the same variant.

X-Sudoku

An X-Sudoku board: the two shaded diagonals must each hold 1–9 as well. The darker centre cell sits on both.

X-Sudoku — also known as Sudoku X, Diagonal Sudoku or X-Factor Sudoku — adds two extra constraints to the Classic puzzle. Same grid, same digits, two more units to satisfy.

The diagonal rules

All standard Classic rules apply — 1–9 exactly once in every row, column and 3×3 box.
The two main diagonals (the highlighted “X”) must also each contain the digits 1–9 exactly once.

Which diagonals count is the question people ask most, so to be precise: only the two corner-to-corner diagonals are constrained — top-left to bottom-right, and top-right to bottom-left. Each is nine cells long. Broken diagonals, off-centre diagonals and the short diagonals inside a box are not units and carry no rule at all.

Everything else follows from that. A cell sitting on a diagonal belongs to four units instead of three, the centre cell belongs to five (row, column, box and both diagonals), and a digit repeated on a diagonal is exactly as illegal as one repeated in a row.

The diagonals are constraints, not hints — they do not make the puzzle harder so much as give you extra ways to deduce. Many solvers find X-Sudoku easier than Classic at the same clue count for precisely that reason.

How to play, step by step

Treat each diagonal as an extra unit. A cell on a diagonal is constrained by its row, its column, its box and the diagonal — four units instead of three.
Start on the diagonals. Because diagonal cells are the most constrained on the board, that's where Naked and Hidden Singles tend to appear first.
Watch the centre cell. The exact middle of the grid lies on both diagonals, making it the single most constrained cell of all.
Use the diagonal for eliminations. Once eight cells of a diagonal are known, the ninth is forced — and that placement often cascades into the surrounding rows, columns and boxes.

Strategy

X-Sudoku rewards diagonal-first thinking. The two extra constraints give you more ways to deduce than Classic, so many solvers find it a fun, slightly different challenge rather than strictly harder. Every Classic technique — naked and hidden singles, naked pairs, X-Wing — works here too; just remember to apply them to the diagonals as well as the rows, columns and boxes. Watch for diagonal pairs, too: a digit confined to just two cells of a diagonal behaves like a pointing pair and can be eliminated from the rest of that diagonal.

Common mistakes

Dropping the diagonals late in the solve. They are full units — a repeat on a diagonal is just as illegal as a repeat in a row.
Confusing which diagonals count. Only the two main corner-to-corner diagonals are constrained; the broken or off-centre diagonals are not.
Forgetting the shared centre. The middle cell lies on both diagonals, so a digit placed there uses up a slot on each.

Best techniques for X-Sudoku

These are the solving techniques that pay off most in X-Sudoku — each links to a full, illustrated guide.

Key Crosshatching The diagonals give two extra lines to crosshatch against. → Key Naked Single The two diagonals add constraints that create even more naked singles than in Classic. → Key Hidden Single in Sudoku The diagonals give each digit two extra units to be hidden-single in. → Key Naked Pair Diagonals are just more units for a naked pair to lock digits out of. → Key Hidden Pair in Sudoku The two diagonals are extra units for a hidden pair to hide in. → Key Naked Triple The diagonals give the triple two more units to lock digits out of. → Key Pointing Pairs in Sudoku (Locked Candidates) Works normally; the diagonals add lines a candidate can be locked to. → Key Box/Line Reduction in Sudoku (Claiming) Unchanged by the diagonals, but they create more lines to reduce from. → Key Snyder Notation Works the same; the diagonals are just more units to apply it in. → Key Full House The diagonals count as units too, so they yield full houses of their own. → Key Hidden Triple in Sudoku The diagonals are extra units for three digits to hide in. → Useful Naked Quad Same as Classic; the diagonals give a few more places to find one. → Useful Hidden Quad in Sudoku Possible; the diagonals add units for four digits to hide in. → Useful Skyscraper Sudoku Technique Explained The diagonals create the strong links a skyscraper needs. → Useful Two-String Kite Diagonals add strong links to build a kite from. → Key X-Wing Sudoku Technique Explained The diagonal constraint often sets up the two-row, two-column rectangle an X-Wing needs. → Key XY-Wing Sudoku Technique (Y-Wing) Explained The diagonals create extra bi-value cells that make good pivots and pincers. → Useful Swordfish Sudoku Technique Explained Possible, though the diagonals often crack the grid with simpler patterns first. → Useful Empty Rectangle Applies as in Classic, with diagonals adding links. → Useful Finned X-Wing Diagonals create more near-fish with fins. → Useful XYZ-Wing Sudoku Technique Explained Diagonals create extra bi-value cells to build wings from. → Useful W-Wing Sudoku Technique Explained Diagonals supply extra strong links. → Useful Remote Pairs Diagonals lengthen the chains available. → Useful Simple Colouring Diagonals extend the conjugate chains. → Useful Turbot Fish The diagonals add strong links, giving the chain extra ways to form. → Useful X-Cycles Diagonal strong links let cycles close that could not on a plain grid. → Useful 3D Medusa Diagonal bi-location links extend the colouring net further. → Useful Grouped X-Cycles The diagonals add group links to draw the loop through. → Useful Multi-Colouring Diagonal conjugate pairs create extra colouring links. → Useful Sashimi Fish Diagonals produce more of these near-fish with an essential fin. → Useful Unique Rectangle Valid, though the diagonals change which rectangles are truly deadly. → Useful X-Chain Diagonals lengthen the chains. → Useful XY-Chain Extra bi-value cells from the diagonals feed the chain. → Useful Forcing Chains The diagonals give chains extra links to travel along. → Useful Aligned Pair Exclusion The diagonals add more cells that two aligned cells can jointly see. → Useful Franken Fish The diagonals add units that make box-based fish more common. → Useful Mutant Fish Extra diagonal units widen the range of mutant fish. → Useful ALS-XZ The diagonals create extra almost locked sets to pair up. → Useful Death Blossom Diagonal units add candidate sets the stem can bloom into. → Useful Alternating Inference Chains (AIC) Diagonal strong links give chains extra rails to travel along. → Useful ALS-XY-Wing The diagonals add candidate sets to serve as the wing's arms. → Useful Kraken Fish Diagonal links give the chain branches extra routes to the target. → Useful Grouped Chains Diagonal groups give the chain more places to turn. →

Difficulties & scoring

This type can be played on all five difficulties. The score multiplier for X-Sudoku is ×1.2 — so every point counts for more on this variant. The "normal time" below is tuned for this type.

Difficulty	Base score	Max (×1.2)	Normal time	Hints
easy	150	360	6 min	5
medium	300	720	11 min	4
hard	600	1440	17 min	3
expert	1000	2400	22 min	2
evil	1500	3600	33 min	1

How the score is calculated

Your score builds up from 0 as you place correct numbers, up to the full base when the board is finished. Then two bonuses are added:

base × your progress × (1 + combo bonus + time bonus) × 1.2

Progress: the live score climbs from 0 to the base as the grid fills with correct numbers.
Combo bonus (up to +50%): keep a clean run for the full bonus. Each error or hint is a combo break that shrinks the bonus — but it never takes Sparks away below the base.
Time bonus (up to +50%): beat the normal time and you earn extra, proportional to how much faster you were. Finishing slower than normal costs nothing — there is no time penalty.
The total is multiplied by ×1.2 and never drops below 0.

Coins

The coins you earn reflect your score — the more Sparks, the more coins (about 1 coin for every 45 Sparks, minimum 1). Spend coins in the Shop on hints and Jokers.

Tip: a clean, hint-free solve under the normal time scores the highest. The Daily Challenge is always Classic and fully scored.