What is a Hidden Single in Sudoku?

If naked singles are the foundation of sudoku strategy, hidden singles are the next floor up. They're the technique that unlocks Medium difficulty — and the one most beginners never learn by name. Here's exactly what a hidden single is and how to find one every time.

The One-Line Definition

A hidden single is a cell where a specific digit can only appear in one place within a row, column, or 3×3 box — even though that cell might appear to have several candidates.

The digit is "hidden" because it looks like the cell has options. But when you scan the rest of the row, column, or box, every other candidate spot for that digit is already blocked. Only one place remains. That's your hidden single.

Why "Hidden"?

The name comes from the fact that the answer is concealed. If you look at the cell in isolation, it appears to have two, three, or more possible candidates. Nothing about the cell itself screams "I have a definite answer." The magic only reveals itself when you ask a different question: not "what can go in this cell?" but "where else in this row (or column or box) can this digit go?"

If the answer is "nowhere else" — you've found a hidden single. The digit has nowhere else to hide in that unit. It has to go here.

A Real Example: Step by Step

Let's look at a concrete puzzle section. We're going to find where the digit 6 goes in the middle-left 3×3 box (rows 4–6, columns 1–3). The box has three empty cells. Two of them are blocked — only one can hold the 6.

Here's the logic:

1. 1
   Pick a digit — in this case, 6. We want to find where 6 goes in the middle-left box (rows 4–6, columns 1–3). The box has several empty cells. We need to eliminate every candidate location except one.
2. 2
   Row 4 is blocked. Row 4 already contains a 6 at column 5 (the middle of the row). One 6 per row — so neither empty cell in row 4 (columns 1 and 2, inside our box) can hold another 6. Both are eliminated.
3. 3
   Row 6 is blocked. Row 6 already has a 6 at column 9 (far right). So both empty cells in row 6 within our box (columns 1 and 2) are eliminated too.
4. 4
   Column 1 is blocked. Column 1 has a 6 at row 2 (top of the grid). So the empty cell at row 5, column 1 is eliminated. That was the last candidate in column 1.
5. 5
   Only one cell remains: row 5, column 3. Row 4 and row 6 are blocked. Column 1 is blocked. Column 2 has a 6 at row 7 — also blocked. That leaves row 5, column 3 as the only cell in the box with no row or column constraint preventing a 6. This is the hidden single. Place the 6 here.

The Three Places to Look for Hidden Singles

Hidden singles can be found in any of the three unit types. You should scan all three:

1. Hidden Single in a Row

Scan across a row and ask: "For digit X, which cells in this row are still open?" If only one cell in the row can hold digit X (because every other empty cell in that row is blocked by X appearing in its column or box), you have a hidden single.

2. Hidden Single in a Column

Same logic, vertical. Scan a column and ask: "Where can digit X go?" If only one empty cell in the column isn't blocked by X appearing in its row or box, that's your hidden single.

3. Hidden Single in a Box

This is the most common and visually satisfying version. Look at a 3×3 box and ask: "Where can digit X go within this box?" Often, row and column constraints from the rest of the puzzle narrow it down to just one cell.

How Hidden Singles Differ from Naked Singles

Both techniques give you a confirmed digit. But the angle of attack is different — and that's what makes hidden singles harder to spot at first.

• Naked single: You're looking at a specific empty cell and finding that only one digit can fit in it. Start with the cell.
• Hidden single: You're looking at a specific digit and finding that only one cell in a unit can hold it. Start with the digit.

Beginners typically get comfortable with naked singles first — they're intuitive. Hidden singles require a mental shift: instead of asking "what goes here?" you ask "where does this go?" Both questions lead to the same kind of answer, but training yourself to ask both makes you dramatically faster.

How Often Do Hidden Singles Appear?

Hidden singles are extremely common — especially in Medium-difficulty puzzles where the board isn't filled enough for naked singles to appear everywhere. Many Medium puzzles require hidden singles to make progress after naked singles are exhausted.

In Hard difficulty, hidden singles often appear together with more advanced techniques. You'll frequently find that unlocking one hidden single opens up a cascade of naked singles — which open up more hidden singles. The puzzle starts to solve itself once you see the first one.

Common Mistakes When Hunting Hidden Singles

• Only checking rows and columns, not boxes. Many hidden singles are box-based. Always check all three unit types.
• Forgetting that a digit can only appear once per unit. The elimination logic only works if you correctly apply the one-per-row, one-per-column, one-per-box rule.
• Stopping after finding one. Finding a hidden single often creates new ones nearby. Rescan after every placement.
• Confusing "candidates" with "confirmed." A cell having digit X as a candidate doesn't mean X goes there. A hidden single is only confirmed when all other candidate cells for that digit in the unit are genuinely blocked.

Using Pencil Marks to Find Hidden Singles

Pencil marks — small candidate numbers written in empty cells — are your best tool for hunting hidden singles efficiently. Here's the workflow:

1. Fill in all candidates for every empty cell on the board.
2. Pick a digit (1 through 9) and scan each row, column, and box for it.
3. If a digit appears as a candidate in only one cell within a unit, that's a hidden single — circle it.
4. Place the digit, update your pencil marks, and repeat.

Sudoku Coach does this automatically — when you tap the hint button, it identifies the technique being used, whether that's a naked single or a hidden single, and explains why the digit goes where it does. You see the reasoning, not just the answer.

Hidden Singles Are the Gateway to Harder Techniques

Once you're fluent with hidden singles, the harder techniques start to make intuitive sense. Techniques like pointing pairs, naked pairs, and eventually X-Wing are all built on the same foundation: finding where a digit can and cannot go within a unit, then using that information to eliminate candidates elsewhere.

Hidden singles are where that reasoning gets its legs. If you understand why a digit must go in the one remaining cell in a box, you're halfway to understanding why two digits locked into two cells can be used to eliminate candidates in an entirely different part of the board.

The progression is logical: naked singles → hidden singles → naked pairs → hidden pairs → pointing pairs. Each builds on the last. And they all start with asking "where can this digit go?"

Practice: Can You Find the Hidden Single?

Consider the top-right 3×3 box of a puzzle. The digits 1, 4, 6, 8, 9 are already placed in five of its nine cells. The four remaining empty cells could each hold 2, 3, 5, or 7. Now ask: where can the digit 3 go within this box? Column 7 already has a 3 elsewhere in the puzzle — so the empty cell in column 7 is blocked. Column 9 also has a 3 elsewhere — blocked. That leaves exactly one empty cell in the box that isn't blocked from holding a 3: the cell in column 8. That's your hidden single — place 3 there with certainty.