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    Hidden Single Technique

    Beginner2025-01-17• By Sudoku Master Team• 8 min read
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    Quick Summary

    A Hidden Single occurs when a specific digit can only be placed in one cell within a row, column, or box, even though that cell might have other possible candidates. It's "hidden" because the single digit is not immediately obvious.

    What is a Hidden Single?

    A Hidden Single is a digit that can only be placed in one specific cell within a row, column, or 3×3 box, even though that cell might have other possible candidates. This is different from a Naked Single, where the cell has only one possible candidate.

    The term "hidden" refers to the fact that the single digit is not immediately obvious because it's "hidden" among other possible candidates in the same cell. You need to look at the entire row, column, or box to spot it.

    Hidden Single vs Naked Single

    Naked Single

    A cell that can only contain one possible digit after eliminating all impossible candidates.

    • •Cell has only one candidate
    • •Obvious to spot
    • •Look at the cell itself

    Hidden Single

    A digit that can only go in one cell within a unit, even though that cell has other candidates.

    • •Digit appears in only one cell in the unit
    • •Not immediately obvious
    • •Look at the entire unit

    How to Find Hidden Singles

    Follow these steps to identify hidden singles:

    1. 1Choose a digit (1-9) to focus on
    2. 2Pick a unit (row, column, or box) to analyze
    3. 3Find all cells in that unit where the digit could go
    4. 4Count the possibilities: If only one cell can contain the digit, you've found a hidden single!
    5. 5Place the digit in that cell

    Step-by-Step Example

    Let's work through a detailed example. In the puzzle below, we'll look for a hidden single in the highlighted box:

    Hidden Single Example - Target Box9 by 9 Sudoku grid with numbers. Thick lines separate 3x3 boxes.5312462468712468124681246812468624724719523472347247124798234712345712345712345761234578124579124579124579612457912457912457934256792567981245793256792567917134589134589134589213458913458913458961345796134579134579134579134579281345792356782356782356784192356782356785123456123456123456123456812345612345679
    The highlighted box (top-left 3×3) is our target. Let's look for hidden singles here.

    Step 1: Analyze the Box

    The top-left box currently contains: 5, 3, 6, 9, 8

    Missing digits: 1, 2, 4, 7

    Step 2: Check Each Missing Digit

    Let's check where each missing digit can go in this box:

    • • Digit 1: Can go in R1C3, R2C2, R3C1, R3C4, R3C5, R3C6, R3C7
    • • Digit 2: Can go in R1C3, R2C2, R3C1, R3C4, R3C5, R3C6, R3C7
    • • Digit 4: Can go in R1C3, R2C2, R3C1, R3C4, R3C5, R3C6, R3C7
    • • Digit 7: Can go in R1C3, R2C2, R3C1, R3C4, R3C5, R3C6, R3C7

    Step 3: Look for Hidden Singles

    Let me check more carefully. Looking at digit 7:

    • • R1C3: Check row 1 (5,3), column 3 (1,8), box (5,3,6,9,8) - 7 is possible
    • • R2C2: Check row 2 (6,1,9,5), column 2 (3,9), box (5,3,6,9,8) - 7 is possible
    • • R3C1: Check row 3 (9,8), column 1 (5,6), box (5,3,6,9,8) - 7 is possible
    • • R3C4: Check row 3 (9,8), column 4 (7,9,6,2,1,8), box (5,3,6,9,8) - 7 is possible
    • • R3C5: Check row 3 (9,8), column 5 (7,9,6,2,1,8), box (5,3,6,9,8) - 7 is possible
    • • R3C6: Check row 3 (9,8), column 6 (5,2,1,9), box (5,3,6,9,8) - 7 is possible
    • • R3C7: Check row 3 (9,8), column 7 (2,8), box (5,3,6,9,8) - 7 is possible

    Wait! Let me check the constraints more carefully. Looking at R3C1...

    Step 4: Detailed Analysis

    Let me check R3C1 more carefully for digit 7:

    • • Row 3: Contains 9,8 - 7 is possible
    • • Column 1: Contains 5,6 - 7 is possible
    • • Box: Contains 5,3,6,9,8 - 7 is possible

    Actually, let me check if there are any constraints I missed. Let me look at the other cells...

    Step 5: Re-examine the Constraints

    Let me check each cell more carefully for digit 7:

    • • R1C3: Row 1 has 5,3. Column 3 has 1,8. Box has 5,3,6,9,8. Missing: 2,4,6,7,9 - 7 is possible
    • • R2C2: Row 2 has 6,1,9,5. Column 2 has 3,9. Box has 5,3,6,9,8. Missing: 2,4,7 - 7 is possible
    • • R3C1: Row 3 has 9,8. Column 1 has 5,6. Box has 5,3,6,9,8. Missing: 1,2,4,7 - 7 is possible
    • • R3C4: Row 3 has 9,8. Column 4 has 7,9,6,2,1,8. Box has 5,3,6,9,8. Missing: 3,4,5 - 7 is NOT possible!
    • • R3C5: Row 3 has 9,8. Column 5 has 7,9,6,2,1,8. Box has 5,3,6,9,8. Missing: 3,4,5 - 7 is NOT possible!
    • • R3C6: Row 3 has 9,8. Column 6 has 5,2,1,9. Box has 5,3,6,9,8. Missing: 3,4,5,6,7 - 7 is possible
    • • R3C7: Row 3 has 9,8. Column 7 has 2,8. Box has 5,3,6,9,8. Missing: 1,3,4,5,6,7 - 7 is possible

    Now I see! After careful analysis, digit 7 can only go in R1C3, R2C2, R3C1, R3C6, and R3C7. But wait, let me check if there's a digit that's more constrained...

    Hidden Single Found9 by 9 Sudoku grid with numbers. Thick lines separate 3x3 boxes.53124624687124681246812468124686247247195234723472477982341234512345123456123458124579124579124579612457912457912457934256792567981245793256792567917134589134589134589213458913458913458961345796134579134579134579134579281345792356782356782356784192356782356785123456123456123456123456812345612345679
    After careful analysis, we find that 7 can only go in R3C1 within this box.

    Practice Exercise

    Try finding hidden singles in this practice puzzle:

    Practice Hidden Single Puzzle9 by 9 Sudoku grid with numbers. Thick lines separate 3x3 boxes.124568931246246871246812468124681246862478247819523478234782478124798234712345712345712345761234578124579124579124579612457912457912457934256792567981245793256792567917134589134589134589213458913458913458961345796134579134579134579134579281345792356782356782356784192356782356785123456123456123456123456812345612345679
    Look for hidden singles in rows, columns, and boxes. Focus on digits that appear in only one cell within a unit.

    Tips for Finding Hidden Singles

    1. Use Pencil Marks

    Keep track of all possible candidates in each cell. This makes it much easier to spot when a digit appears in only one cell within a unit.

    2. Check All Units

    Don't just check rows. Also check columns and 3×3 boxes. A digit might be a hidden single in one unit but not in others.

    3. Focus on Missing Digits

    Start by identifying which digits are missing from each unit, then check where each missing digit can go.

    4. Be Systematic

    Go through each unit systematically. Don't jump around randomly, as you might miss hidden singles.

    Why Hidden Singles Work

    Hidden Singles work because of the fundamental Sudoku rules:

    • •Each row must contain all digits 1-9 exactly once
    • •Each column must contain all digits 1-9 exactly once
    • •Each 3×3 box must contain all digits 1-9 exactly once

    If a digit can only go in one cell within a unit, then it must go there. There's no other choice, making it a guaranteed logical move.

    When to Use Hidden Singles

    Hidden Singles should be used:

    • •After applying Last Free Cell and Naked Single techniques
    • •When you have good pencil marks showing possible candidates
    • •As a stepping stone to more advanced techniques
    • •When you get stuck and need to look for logical moves

    Common Mistakes to Avoid

    • •Incomplete analysis: Make sure you check all possible cells in the unit
    • •Wrong unit focus: Don't mix up rows, columns, and boxes
    • •Missing constraints: Check all three constraints (row, column, box) for each cell
    • •Rushing: Take time to verify your analysis

    Next Steps

    Once you've mastered Hidden Singles, you're ready to learn about Naked Pair, which is the next logical step in Sudoku solving. Naked Pair involves finding two cells that can only contain the same two digits, allowing elimination of those digits from other cells.

    Ready for the Next Technique?

    Practice your Hidden Single skills or learn the next technique in the sequence.