Naked single
12345789{6}
→123457896
Eight digits are excluded by the row, column, or box. Candidate 6 is the exact resulting move.
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Sudoku Board & Controls
Time: 00:00
Mistakes: 0 (unlimited)
Hints: 0
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Today’s puzzle is ready. Select an empty cell and enter a digit.
GivenYour entryNotesHintIncorrect
Puzzle quality: Starlight’s Logic Grade pipeline checks each curated source grid for exactly one solution, then rates it by the hardest required logical technique rather than clue count. Offered puzzles are solvable without guessing. Sudoku entries and progress stay in this browser.
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How to Play Sudoku
Sudoku is a logic puzzle, not an arithmetic problem—no mathematical formula is needed. Fill the grid using three rules:
- Each row contains 1–9 once.
- Each column contains 1–9 once.
- Each 3×3 box contains 1–9 once.
Use this interface
Select a cell, then use the keypad or keys 1–9. Turn on Notes for candidates; Erase or Backspace clears. Orange underlining marks duplicate conflicts, while red marks a digit that differs from the unique solution. Immediate checking is optional; Check Board is always available. Mistakes are unlimited.
Keyboard: arrows move one roving focus; N toggles notes, P pauses, and Ctrl/⌘+Z / Ctrl/⌘+Y undo and redo.
How difficulty is determined
Easy puzzles use singles. Medium adds hidden singles. Hard adds locked candidates and pairs. Expert puzzles require techniques beyond that set. Every puzzle is checked for one solution.
Rules and strategy reviewed 15 July 2026 by the Starlight Robotics tools development team.
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Sudoku Strategy: From First Move to Advanced Logic
Begin by scanning rows, columns, and boxes for cells with one candidate. Refresh candidate notes after each placement. If stuck, look for a digit that has only one possible position in a unit before moving to eliminations.
Beginner strategy and what to do when stuck
A naked single is a cell with one candidate. A hidden single is a digit with only one available cell in a row, column, or box. Re-scan after every move, remove stale notes, and use the explanatory Hint control for a logical next step.
Advanced techniques
Naked pairs lock two candidates into two cells, allowing those digits to be removed elsewhere in the unit. Box-line interactions eliminate a candidate outside a box when every occurrence inside that box lies on one row or column. Expert grids may also use triples, X-Wings, or chains.
Worked Sudoku Examples
Hidden single
1224371545382989{6,9}
→12243715453829896
In this box, 6 appears as a candidate only in the last cell, so the other candidate 9 is eliminated and 6 is placed.
Naked pair
{2,7}{2,7}2,5,71,53,84,76,91,94,5
→2,72,751,53,846,91,94,5
The pair {2,7} owns the first two cells. Eliminate 2 and 7 from the other cells; the third cell becomes 5.
Box-line interaction
{4}{4}
→44no 4no 4no 4
All 4s in the box lie on its middle row. Eliminate 4 from that row outside the box; the next single can then emerge.
Sudoku FAQ
Is this online Sudoku free?
Yes. Every puzzle and game control is free to use.
Do I need to sign up or install anything?
No. The game runs in a web browser without an account or installation.
Is my Sudoku progress saved?
Yes. The active puzzle, entries, notes, elapsed time, mistakes, difficulty, and settings are saved in this browser using local storage.
How do candidate notes work?
Turn on Notes, then choose digits to add or remove small candidate marks. Auto Notes fills every currently legal candidate.
How do Sudoku hints work?
Hints first explain a naked single or hidden single. If neither is available, the game clearly identifies a correct cell and digit as a reveal.
How are mistakes counted?
A mistake is counted when immediate checking identifies a wrong final digit, or when Check Board finds a wrong digit not counted before. Mistakes are unlimited.
Does every puzzle have one solution?
Yes. Every source puzzle is checked by a two-solution solver and only puzzles with exactly one solution are offered.
How is difficulty rated?
Difficulty is based on the hardest logical technique the grader needs, including singles, hidden singles, locked candidates, and pairs—not simply the number of clues.
Can I pause or restart a puzzle?
Yes. Pause stops the timer and hides the board; Restart returns the current puzzle to its original clues after confirmation.
Does Sudoku work on mobile and with a keyboard?
Yes. The board and keypad are responsive. Keyboard players can use arrow keys, digits, Backspace, N for notes, P to pause, and Ctrl or Command plus Z or Y for undo and redo.