Sudoku X

American architect Howard Garns created the puzzle that appeared as Number Place in 1979, years before the name Sudoku became familiar. Sudoku X adds one sharp twist: each of the two corner-to-corner diagonals must also hold 1 through 9 exactly once. Research published in 2019 found an association between frequent number-puzzle use and better cognitive performance among adults aged 50 to 93, though the observational study did not show that puzzles caused the difference. The extra diagonal logic creates fresh deductions at every level. Choose a difficulty and play Sudoku X online now.

1. Choose Easy, Medium, or Hard. The board has 81 cells in nine rows, nine columns, and nine outlined 3x3 boxes. Fixed digits are clues and cannot change. Colored corner marks identify the two long diagonals.
2. Select an empty cell. Remove every digit already present in its row, column, and box from consideration. For a diagonal cell, also remove digits used on that diagonal. The center belongs to both diagonals, so it must satisfy five regions at once.
3. Enter a digit with the controls or 1 to 9 keys. Turn on Notes, or press N, to record candidates. Red entries need another look. Delete clears an editable cell, Undo restores the previous action, and Hint supplies one correct entry.
4. Scan again after every placement because one digit can change a row, box, and diagonal together. Finish when each required group contains 1 through 9 with no repeats. The completion message records your time and moves. Begin on Easy if the diagonal rule feels new.

• Every row contains the digits 1 through 9 exactly once. A repeated digit makes the row invalid, even when the two copies sit in different 3x3 boxes.
• Every column contains the digits 1 through 9 exactly once. Row and column restrictions apply to every cell, including cells on a marked diagonal.
• Each of the nine 3x3 boxes contains 1 through 9 exactly once. The heavier lines separate these boxes from ordinary cell boundaries.
• The main diagonal from the upper-left corner to the lower-right corner contains 1 through 9 exactly once. The opposite diagonal from the upper-right corner to the lower-left corner follows the same rule.
• The center cell sits on both long diagonals. Its one digit counts in its row, column, box, main diagonal, and opposite diagonal.
• Given clues stay fixed. Each generated puzzle retains one solution under the full row, column, box, and diagonal rule set.

• 1. Scan each diagonal as a separate nine-cell region. List the missing digits, then test their possible cells against crossing rows, columns, and boxes. A digit with one legal diagonal position is a hidden single, even when that cell still appears flexible from its row alone.
• 2. Use diagonal-box intersections. Three cells from a long diagonal pass through a corner or center box. If a missing digit can only occupy those diagonal cells inside that box, remove the digit from the box's other six cells. Reverse the check when box restrictions confine a candidate to one diagonal cell.
• 3. Recheck the center early. Row 5, column 5, the center box, and both diagonals meet at r5c5. Build its candidate set from all five regions. Even when it remains unresolved, every eliminated center candidate can sharpen scans across two diagonal directions at once.
• 4. Mark diagonal pairs precisely. If two cells on one diagonal contain the same two candidates, those digits cannot appear elsewhere on that diagonal. Then inspect the rows and columns crossing the pair. One of those intersections may become a single or create a useful box-line reduction.
• 5. Compare both ends of a diagonal. For one missing digit, scan inward from each corner and reject cells blocked by their rows or columns. This bidirectional cross-hatching is faster than checking nine cells randomly, and it often exposes a forced position near the center box.

• Sudoku: Standard Sudoku uses the same 9x9 grid, row rules, column rules, and 3x3 boxes, but its long diagonals may repeat digits. It offers the clearest route for learning singles, candidate notes, cross-hatching, pairs, and box-line reductions before adding diagonal constraints.
• Killer Sudoku: Killer Sudoku adds outlined cages with target sums. It combines placement logic with arithmetic combinations, while Sudoku X adds two all-different regions instead. Both reward careful candidate tracking, but the information arrives in different forms.
• Hyper Sudoku: Hyper Sudoku adds four shaded 3x3 regions inside the standard grid. Like the X rule, these extra regions create more intersections and new hidden singles while preserving normal rows, columns, and boxes.

• The two marked diagonals are easy to understand, yet they change where useful information appears. Players must look beyond the nearest box and treat each corner-to-corner line as a full region.
• Diagonal cells often produce satisfying chain reactions. One center placement can reduce candidates in five regions, while a corner placement can affect a row, column, box, and diagonal together.
• Three difficulty settings support short practice sessions and longer solves. Notes, undo, mistake feedback, keyboard input, and hints let players choose how much assistance they want.
• The objective stays measurable. Every placement either respects or violates a known set of constraints, and the final grid can be checked directly without subjective scoring.

Play Diagonal Sudoku free online directly on this page. No account or download is required. Pick a level, follow the colored diagonal markers, and use notes to manage candidates. Try to solve from logic first, then use a hint if one position stops your progress. The move counter and timer make repeat sessions easy to compare. Start a Sudoku X puzzle now and see how quickly the extra two regions reshape familiar Sudoku decisions.

Sudoku X turns two familiar lines into powerful solving regions. Scan each diagonal for missing digits, exploit box intersections, and give the center cell special attention. Easy, Medium, and Hard grids provide room to build those habits at your own pace. Select an empty cell, switch on notes when candidates accumulate, and begin your free online Diagonal Sudoku puzzle. Every correct deduction brings four standard region types and two diagonal paths closer to completion.

• Number puzzles and cognitive function in older adults, International Journal of Geriatric Psychiatry, 2019
• Sudoku, Encyclopaedia Britannica