Naked Singles
A cell with only one candidate must take that value. These often appear after you pencil mark possibilities or place a neighboring digit.
Example: Row 4 is missing just a 5 in column 7, so place 5 there immediately.
Ruedoku — Free Sudoku with Teaching Mode
Tap a cell then choose a number · Shift + number toggles candidates
How to Play Sudoku
New to Sudoku? Follow these quick steps to build momentum. Each puzzle starts with “givens” (fixed digits). Your job is to fill the remaining squares without repeating numbers in any row, column, or 3×3 box.
Step 1: Understand the goal
Every row, column, and 3×3 box must contain the digits 1 through 9 exactly once. Givens are fixed, everything else is yours to solve.
Step 2: Scan for forced numbers
Look across each row, column, and box for cells that already have eight digits placed. The final missing digit is a guaranteed placement.
Step 3: Use pencil marks
Toggle candidates in Ruedoku to track which numbers are still possible in a cell. This keeps complex grids organized and prevents guesswork.
Step 4: Cross-hatch boxes
When a digit can only appear in one row or column within a box, note that restriction. It will often unlock placements in intersecting units.
Step 5: Repeat and reassess
After every placement, rescan the puzzle. Fresh singles and new candidate eliminations appear quickly when you keep the board tidy.
Sudoku Solving Techniques
Improve your solve rate with proven strategies. Ruedoku hints explain each move and Auto Play demonstrates how these techniques unfold on real puzzles.
Hidden Singles
When a digit is the only candidate within a unit (row, column, or box), it is hidden among other marks. Clear the noise and place it with confidence.
Example: In box 6, only one cell allows a 9, so the 9 belongs to that square.
Locked Candidates
If a candidate is restricted to one row or column inside a box, eliminate that digit from the rest of that row or column outside the box. Ruedoku hints call this out as “pointing pairs”.
- Locked candidates come in two flavors: pointing (box to line) and claiming (line to box).
- Use them early to reduce clutter before searching for advanced patterns.
Example: If 3s only appear in the top row of a box, remove 3 from other cells in that row.
Naked Pairs & Triples
Two cells that share the same two candidates (or three cells sharing three candidates) lock those digits to those cells. Remove the digits from every other cell in that unit.
- Pairs and triples work in rows, columns, and boxes—check all three after you spot one.
- If a pair spans a box edge, revisit the intersecting unit for hidden singles.
Example: Two cells in column 5 can only be 2 or 8; eliminate 2 and 8 from the rest of column 5.
Box-Line Interactions
Also called pointing pairs or claiming. If a digit is confined to one line within a box, remove it from the rest of that line. Conversely, if a digit is restricted to one box within a line, clear it from the remainder of the box.
- Trace the candidate across both the box and the line to ensure the restriction really exists.
- Combining this with naked pairs frequently opens up new singles.
Example: All the 7s in row 2 fall inside the left-middle box, so remove 7 from the other cells in that box.
X-Wing
Spot rectangles where a digit appears in exactly two cells of two different rows (or columns). Those four corners form an X-Wing and let you eliminate the digit from the intersecting columns or rows.
- X-Wings come in both row-based and column-based forms—scan candidates in both directions.
- Candidate highlighting tools make it easier to confirm the four-corner rectangle before you eliminate digits.
Example: If the digit 4 appears in exactly two cells on rows 1 and 5 sharing columns 3 and 8, remove 4 from all other cells in columns 3 and 8.
Difficulty Guide
Easy: Solvable with singles and straightforward scanning.
Medium: Expect locked candidates and the occasional naked pair.
Hard: Requires layered candidate work plus techniques like box-line interactions.
Ultra Hard: Minimal clues with advanced patterns such as X-Wings. Mastering pencil marks keeps these beasts manageable.
Sudoku FAQ
- Is Ruedoku free to play?
- Yes. No account, no payment, no ads blocking the game. Play as many puzzles as you like.
- What difficulty levels are available?
- Four levels: Easy, Medium, Hard, and Ultra Hard. Easy is solvable with basic scanning. Medium introduces naked pairs. Hard requires locked candidates. Ultra Hard features minimal clues and advanced patterns like X-Wings.
- What is teaching mode?
- Teaching mode shows hints that explain the technique used to place each digit — not just the answer, but why it goes there. Designed to help you learn solving strategies while you play.
- Can I use pencil marks?
- Yes. Toggle Candidates to jot possible digits in each cell. On keyboard: hold Shift while typing a number to toggle a candidate mark. Useful for harder puzzles where multiple possibilities need tracking.
- What is Auto Play?
- Auto Play solves the puzzle step by step, using the same techniques the hint system explains. Useful for learning advanced patterns or seeing how a difficult puzzle unfolds.
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