Hidden Single in Sudoku
A hidden single is one of the most important early Sudoku techniques because it teaches digit placement thinking, not just cell-level possibilities. The answer may not look obvious in one cell, but it becomes clear when you scan the full row, column, or box.
Summary
Use unit-wide scanning to find numbers that can only fit in one position.
Table of Contents
What this technique is
A hidden single occurs when one digit has only one legal position in a row, column, or box, even if the target cell still looks ambiguous.
When to use it
Use hidden singles when obvious moves slow down and you need to place a specific digit by scanning the whole unit.
How it works
Pick one digit, inspect all open cells in a unit, and eliminate blocked positions. If one position remains, place the digit.
Step-by-step
- Choose a row, column, or box.
- Track one missing digit across all open cells.
- Eliminate blocked positions and place if one remains.
Example
In a 3x3 box, candidate 7 may appear possible in three cells at first. Row/column overlaps can block two, revealing one legal spot.
Example
A box may be missing digit 7 with three candidate cells. If two are blocked by nearby 7s in overlapping rows/columns, the remaining cell is a hidden single.
Common Mistakes
- Searching cell by cell instead of digit by digit.
- Looking only at one cell instead of the full unit.
- Forgetting that the same logic applies to rows, columns, and boxes.
- Confusing hidden singles with naked singles.
- Missing hidden singles after a new placement changes nearby candidate availability.
- Failing to re-scan after each successful placement.
FAQ
What is a hidden single in Sudoku?
A hidden single is a digit that has only one valid position in a row, column, or 3x3 box.
Is hidden single a beginner technique?
Yes. It is a core beginner technique that appears often in easy and medium puzzles.
How is hidden single different from naked single?
A naked single is obvious from one cell; a hidden single appears after scanning the whole unit.