A hidden single occurs when a candidate can only fit in one cell within a unit (row, column, or 3x3 block), even if that cell still shows other possible candidates. This is different from a naked single, where a cell has only one candidate remaining.

Hidden singles are one of the most important Sudoku strategies for beginners. They appear in nearly every easy or medium-level Sudoku puzzle and are often the key to making steady progress. By identifying them early and often, you can place numbers confidently and build a foundation for more advanced techniques, such as hidden pairs, when you play Sudoku.

How to Find Hidden Singles

Hidden singles must meet the following criteria:

• The candidate number can only appear in one cell within a row, column, or 3x3 block.
• The cell must contain other candidates, effectively “hiding” the single candidate.
• That number is not a valid candidate in any other cell within the unit.

To find hidden singles, follow these simple steps:

1. Note all possible candidate numbers in each cell. You can analyze cells and place pencil marks in them by turning on candidate mode (check the box next to “This number is a possibility”) or quickly populate each cell’s possibilities by clicking the “Show Possibilities” button.
2. Find a candidate that only appears once in a row, column, or block, even if that cell contains other candidates. For example, when looking at row 1, you can see that 8 only appears in cell H1, even though H1 has multiple candidates.
3. Confirm it cannot be placed elsewhere in the unit. If a number can only go in one cell within that unit, it must be placed there, even if that number appears as a candidate in intersecting units. For example, 8 cannot be placed anywhere else in row 1.
4. Place the hidden single. Because 8 only appears in H1 for that row, it must be the answer for that specific cell (H1).

Hidden singles often appear after placing a number and updating candidates, so make sure to rescan units for new hidden singles regularly.

Hidden Single Examples

Although many consider it a basic technique, finding a hidden single can create a chain reaction of placements. So using this simple Sudoku strategy can often unlock a puzzle quickly and level up your puzzle-solving skills if you can find hidden singles in rows, columns, and blocks.

Hidden Singles in a Row

To find a hidden single in a row:

1. Note all possible candidate numbers in the grid. Click the “Show Possibilities” button to do this in one move or add pencil marks using candidate mode.
2. Look for a candidate that only appears once in a row of the puzzle, even if that cell contains other candidates. For example, when looking at row 4, you can see that 3 only appears in cell D4.
3. Confirm it cannot be placed elsewhere in the row. Candidate 3 is not a valid placement for any other cell in that row (4).
4. Place the hidden single. Because 3 only appears in D4 for that row, it must be the answer.

Hidden Singles in a Column

To find a hidden single in a column:

1. Note all possible candidate numbers in the grid. Click the “Show Possibilities” button or add your own pencil marks using candidate mode.
2. Look for a candidate that only appears once in a column of the puzzle, even if that cell contains other candidates. For example, when looking at column C, you can see that 4 only appears in C5 and 3 only appears in cell C7.
3. Confirm it cannot be placed elsewhere in the column. Candidates 3 and 4 don’t appear in any other cells in column C.
4. Place the hidden single. In this column, 4 can only go in C5 and 3 can only go in C7, so place them there, then remove those numbers as candidates from intersecting units.

Hidden Singles in a Block

To find a hidden single in a block:

1. Note all possible candidate numbers in the grid. You can view them by clicking the “Show Possibilities” button or by adding pencil marks with candidate mode.
2. Look for a candidate that only appears once in a block of the puzzle, even if that cell contains other candidates. For example, when looking at the bottom-right block, you can see that 1 only appears in cell G7.
3. Confirm it cannot be placed elsewhere in the block. No other cell in the block has 1 as a candidate.
4. Place the hidden single. Because 1 only appears in G7 for that block, it must be the answer and can be placed in that cell.

Finding and placing hidden singles is a reliable solving technique that can create a cascade of placements, allowing you to move quickly through easy Sudoku puzzles. When done proficiently, you can elevate your Sudoku solving skills and try harder puzzles and more advanced techniques when you play Sudoku online.