Hidden Single Sudoku Technique Explained

The essential companion to Naked Singles

Hidden Single example in Sudoku

What is a Hidden Single?

A Hidden Single is one of the two fundamental solving techniques in Sudoku, complementing the Naked Single. It occurs when a candidate number can only appear in one cell within a unit (row, column, or 3×3 box), even though that cell might contain other candidates as well.

The number is "hidden" amongst other possibilities but is the only place where that particular number can go in that unit. Unlike Naked Singles where a cell has only one candidate, Hidden Singles are found by analysing where a specific number can be placed within a row, column, or box.

The Uniqueness Rule: Each row, column, and 3×3 box must contain each number (1-9) exactly once. If a number can only fit in one cell within a unit, it MUST go there, regardless of what other candidates that cell contains.

Hidden Single vs Naked Single

Naked Single

Cell perspective: "This cell has only one candidate"

Question: "What can go here?"

Example: Cell has candidates {5} only

Hidden Single

Unit perspective: "This number can only go in one cell"

Question: "Where can this number go?"

Example: Cell has {2,5,7} but 5 is hidden single for the row

The Three Types of Hidden Singles

Row Hidden Single

A number can only appear in one cell within a row

Column Hidden Single

A number can only appear in one cell within a column

Box Hidden Single

A number can only appear in one cell within a 3×3 box

How to Find Hidden Singles

Step-by-Step Process:

  1. Choose a unit to analyse (a row, column, or 3×3 box)
  2. Select a missing number from that unit (one that hasn't been placed yet)
  3. Check each empty cell in that unit to see if the number is a valid candidate
  4. Count the possibilities – how many cells can contain this number?
  5. If only one cell is possible – you've found a Hidden Single! Place the number.
  6. If multiple cells are possible – this number is not a Hidden Single in this unit
  7. Repeat for all missing numbers in the unit
  8. Move to the next unit and repeat the process

Why Hidden Singles Are Crucial

  • Most common technique: Hidden Singles appear more frequently than Naked Singles in most puzzles
  • Solves easier puzzles entirely: Easy and many medium puzzles can be solved using only Naked Singles and Hidden Singles
  • No pencil marks required: You can find Hidden Singles by visual scanning without writing candidates
  • Creates solving momentum: Each Hidden Single placed often reveals several more
  • Foundation for advanced play: Understanding Hidden Singles trains the unit-based thinking needed for advanced techniques

Systematic Scanning Strategies

Strategy 1: Unit-by-Unit (Recommended for Beginners)

  1. Choose row 1 as your starting point
  2. List missing numbers: e.g., {1, 4, 6, 9}
  3. For each missing number: Where can it go in this row?
  4. Move to row 2 and repeat
  5. After all rows: scan all columns, then all boxes

Strategy 2: Number-by-Number (Advanced)

  1. Choose number 1 to track across entire grid
  2. Check each row: Does this row need a 1? Where can it go?
  3. Check each column: Does this column need a 1? Where can it go?
  4. Check each box: Does this box need a 1? Where can it go?
  5. Move to number 2 and repeat for the entire grid
Pro Tip: Experienced solvers often combine both strategies, using unit-by-unit for partially filled units and number-by-number for numbers that appear frequently. Find the approach that works best for your solving style!

Common Mistakes to Avoid

  • Forgetting to check all three constraints: A number must be valid for the row, column, AND box
  • Stopping too early: Check ALL empty cells in the unit before concluding
  • Confusing with Naked Singles: Hidden Singles focus on where a NUMBER can go, not what a CELL can contain
  • Missing box Hidden Singles: Boxes are often overlooked but frequently contain Hidden Singles

Conclusion

Hidden Singles represent a fundamental shift in Sudoku thinking – from cell-focused to unit-focused analysis. Whilst Naked Singles ask "What can go in this cell?", Hidden Singles ask "Where can this number go in this unit?" Mastering both perspectives is essential for efficient Sudoku solving.

Together with Naked Singles, Hidden Singles form the foundation of all Sudoku solving. Every advanced technique ultimately works by creating situations where Hidden or Naked Singles can be found. Master these two techniques, and you'll have the tools to solve most easy and medium puzzles whilst building the analytical skills needed for harder challenges.

Ready to Practice?

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