LeafyOrbSudoku techniques · Easy level
Hidden singles: the digit with only one home
A hidden single is a digit that has only one possible cell left in a row, column or box. That cell must take it, even if it could still hold other digits.
How it works
Every row, column and box needs each digit from 1 to 9 exactly once. So instead of asking "what can go in this cell?", ask "where can the 7 go in this box?" If the answer is one cell, you have found a hidden single.
It is called hidden because the cell itself may still look open: it might allow 2, 7 and 9. The single only shows when you look at the digit, not the cell. After the naked single, it is the most common move in every Sudoku, and on Easy puzzles it is usually all you need.
- The row, column or box the move uses
- The cells that make the pattern
- Candidates the move removes
Example 1 · Box · from Easy puzzle #5
Look for the 1 in box 5. Every other empty cell there already sees a 1 in its own row, column or box, so 1 can only go in R5C4, even if R5C4 could still hold other digits.
NG-18N6GZ-SWRHI6-6N08BV-IXTTU9-01BTST-91GDA1-AZ35HK-BXOHOF-R4J5B
Cells are named by row and column: R4C3 is row 4, column 3. Boxes are numbered 1 to 9, left to right and top to bottom.
How to spot it
- Pick a digit, say 1, and a box that does not have it yet.
- Cross-hatch: follow every 1 already on the board along its row and column, and block the cells it passes through in your box.
- If only one empty cell in the box is left unblocked, the 1 goes there.
- Do the same for rows and columns, then move to the next digit. Going 1 to 9 in order means you never miss one.
More examples
Example 2 · Row · from Easy puzzle #75
Look for the 5 in row 6. Every other empty cell there already sees a 5 in its own row, column or box, so 5 can only go in R6C8, even if R6C8 could still hold other digits.
NG-1WSL4C-4JFU8O-BK2MVT-JKWARG-ML2W8F-XCMAGW-UXEFRX-R4E0C5-6Q8UO
Example 3 · Column · from Easy puzzle #77
Look for the 6 in column 6. Every other empty cell there already sees a 6 in its own row, column or box, so 6 can only go in R6C6, even if R6C6 could still hold other digits.
NG-18N6VQ-B2WAMA-UVWJXU-TSTJH5-2S3JRW-W9WO5C-O4ZYI3-WZ5B0Y-1FQ3Q
Common mistakes
- Forgetting the box. A cell can be blocked by a digit in its row, its column or its box.
- Stopping at the first box. Most hidden singles are found by sweeping one digit across the whole board.
- Mixing it up with a naked single, which is the other way round: a cell with one possible digit.
Questions
What is the difference between a naked single and a hidden single?
A naked single is a cell that can only hold one digit. A hidden single is a digit that can only go in one cell of a row, column or box. The first looks at a cell, the second looks at a digit.
Do I need pencil marks to find hidden singles?
No. Cross-hatching from the digits already on the board is enough, which is why hidden singles are the first thing to look for when you open a new puzzle.
Which Ninebloom level uses hidden singles?
Easy. Beginner puzzles need only naked singles, and Easy adds hidden singles. Every level above uses both.