Beginner technique

Hidden Single Sudoku – Find the Only Place

Learn Hidden Singles in Sudoku: track one digit across a row, column, or box, find its only legal position, and avoid confusing them with Naked Singles.

Practice on Medium Sudoku Read guide
Interactive technique example

See Hidden Single on a real grid

Read the candidate pattern first, then reveal the logical action. This example uses the same candidate rules as the site difficulty grader.

Technique detected: Hidden Single Find the highlighted pattern before revealing the move.
Why this move?

2 can appear in only one cell of row 1, even though r1c3 still has several pencil marks.

Action: Place 2 in r1c3.

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Pattern Logical result
Core idea

What is a Hidden Single?

A Hidden Single appears when a digit has only one legal position left inside a row, column, or 3×3 box. The target cell may still show several pencil marks, which is why the single is “hidden” among other candidates.

Instead of asking “what can go in this cell?”, ask “where can this digit go in this unit?”. That change of viewpoint is the key difference between Naked and Hidden Singles.

Scanning

Scan one digit across a unit

Choose a missing digit in a row, column, or box and mark every cell where that digit is blocked by the crossing units. If only one location survives, that location must contain the digit even if the cell has other candidate marks.

Boxes are especially useful for beginners because crossing rows and columns can eliminate several positions at once. Repeat the same process for different digits before adding more pencil marks.

  • Pick one missing digit, not one empty cell.
  • Eliminate positions using the two crossing units.
  • Confirm that exactly one position remains in the unit.
Compare

Hidden Single vs Naked Single

A Naked Single is forced because one cell has one candidate. A Hidden Single is forced because one digit has one location in a unit. The final move may be identical, but the evidence you use to prove it is different.

This distinction matters when you review hints or learn from a solver. If a cell still contains three pencil marks but one of those digits can appear nowhere else in the row, you have a Hidden Single, not a Naked Single.

Next step

Move from Singles to candidate structure

After Singles stop producing progress, build a clean candidate map. Candidates let you see structures that are impossible to notice from givens alone, including Locked Candidates and Naked or Hidden Pairs.

Do not treat pencil marks as decoration. They are the working data for every technique that follows.

Apply it immediately on a real puzzle

A technique becomes automatic only after you use it across many different boards. Play at a manageable level, prioritize certain deductions, and review the step that caused you to get stuck.

Practice on Medium Sudoku
Learn by technique

Sudoku solving guide cluster

How to Solve Sudoku

Each guide below targets a distinct search intent and skill, from a beginner workflow to locked candidates, pairs, and X-Wing. Follow them in order if you are unsure where to start.

How to solve Sudoku: techniques from basic to advanced A systematic Sudoku solving path: scan singles, manage candidates, use pairs, locked candidates, and advanced patterns without guessing. Sudoku Tips for Beginners: Solve Faster with Fewer Mistakes Practical Sudoku tips for beginners: scan regions, choose good starting points, use Notes at the right time, and avoid habits that make easy games confusing. Naked Single Sudoku – Find Forced Cells Fast Learn how to spot a Naked Single in Sudoku, verify the only candidate left in a cell, and turn forced cells into a reliable solving loop. Sudoku Candidates & Pencil Marks – Practical Guide Use Sudoku candidates and pencil marks accurately: calculate legal digits, keep notes updated, and prepare for Locked Candidates, Pairs, and X-Wing. Locked Candidates in Sudoku: Pointing & Claiming Explained Understand locked candidates in Sudoku through pointing and claiming, how to identify the pattern, eliminate candidates in the right places, and avoid common mistakes. Naked & hidden pairs in Sudoku: how to identify and eliminate digits Learn the difference between naked and hidden pairs in Sudoku, how to use two candidates for safe eliminations, and the signs that make pairs easier to spot. What Is X-Wing in Sudoku? How to Spot and Use It Correctly A step-by-step X-Wing Sudoku guide: four-corner conditions, row/column scanning, candidate elimination, and patterns that are easy to misread.