Hidden Singles: Unmasking the Lone Candidate

Hidden Singles are the conceptual inverse of Obvious Singles. While an Obvious Single reveals itself because a cell has only one candidate, a Hidden Single lurks inside a cell that appears to have multiple candidates. The secret: within the cell's row, column, or 3×3 box, one of its candidates appears nowhere else. That candidate is the only possible home for that number in the entire unit, and therefore the cell must contain it. It's "hidden" because you have to look beyond the cell's own candidate list and survey the entire unit to find it.

How Hidden Singles Work

The process requires scanning candidates across an entire unit:

• Step 1: Choose a row, column, or 3×3 box to analyze.
• Step 2: For each number 1 through 9, count how many cells in that unit list the number as a candidate. Use pencil marks or the app's candidate view to scan efficiently.
• Step 3: If any number appears as a candidate in exactly one cell of the unit, you've found a Hidden Single. That number must go in that cell — there is no other place for it.
• Step 4: Place the number and remove it as a candidate from all connected cells. Crucially, also remove all other candidates from the solved cell — the hidden single was hiding among them, and now that the cell is solved, those other candidates are eliminated too.

A Concrete Example

Consider the top-left 3×3 box. You scan through the box, checking each number's candidate distribution. For the number 6, you find the following:

• R1C1: candidates 9
• R1C2: candidates 9
• R1C3: candidates 8
• R2C1: candidates 9
• R2C2: candidates 8
• R3C1: candidates 9

The number 6 appears as a candidate only in R1C1. No other cell in this 3×3 box can contain a 6. Therefore, even though R1C1 also lists 2 and 9 as candidates, the cell must be 6. We place 6 in R1C1, then eliminate 2, 6, and 9 as candidates from all cells that share a unit with R1C1. The 6 was hidden among the 9 candidates — but from the box's perspective, it was the only option for 6.

Hidden Singles vs. Last Possible Number

Astute readers may notice that Hidden Singles strongly resemble the Last Possible Number technique — and they should. Hidden Singles are essentially Last Possible Number applied with full candidate notation. The difference: beginners use Last Possible Number by visually scanning the board for placed numbers. Hidden Singles formalize this by checking candidate lists systematically. When you find a Hidden Single on a Medium puzzle, you're performing Last Possible Number with the precision that comes from complete candidate data.

When to Use Hidden Singles

Hidden Singles are indispensable on Medium and Hard puzzles. After placing the "obvious" moves using Obvious Singles, you'll invariably reach moments where no cell has a single candidate — yet the puzzle is far from solved. This is the Hidden Single's moment. By scanning each unit methodically, you'll uncover numbers that have only one possible home. Each Hidden Single placement removes that number from the rest of the unit, often creating new Obvious Singles in the process.

On Expert and Extreme puzzles, Hidden Singles appear alongside Hidden Pairs and Obvious Triples. A common solving workflow is: use Obvious Singles until stuck, scan for Obvious Pairs and Triples to eliminate candidates, then re-scan for Hidden Singles revealed by those eliminations. This cycle — eliminate, reveal, place — is the heartbeat of advanced Sudoku solving.