What is a Naked Single in Sudoku?

A naked single is the very first strategy every sudoku solver needs to understand. Once you see it, you'll never look at a puzzle the same way. Here's exactly what it is and how to spot it every time.

The One-Line Definition

A naked single is a cell where only one number can possibly go — because every other number from 1–9 is already present in the same row, column, or 3×3 box.

It's "naked" because the answer is out in the open. There's no hiding it. No complex logic required. You just have to look.

A Real Example

Let's look at a specific cell and walk through the logic step by step. Imagine this is a portion of your puzzle, and we're trying to figure out the empty cell in row 5, column 5:

How to Solve It: The Elimination Method

Here's how to find a naked single systematically. We eliminate numbers that can't fit in our cell:

1. 1
   Check the row. Row 5 already contains: 4, 8, 3, 1. Cross those off your list of possibilities.
   Remaining: 2, 5, 6, 7, 9
2. 2
   Check the column. Column 5 already contains: 7, 9, 6, 2, 1, 8. Cross those off too.
   Remaining: 5
3. 3
   Only one number left. After checking the row and column (and box if needed), only 5 remains. That's your naked single.

Why "Naked"?

The terminology comes from the broader family of "naked" and "hidden" techniques in sudoku. A naked technique is one where the answer (or constraint) is immediately visible in the cell itself. You can see the candidate is alone — it's out in the open.

Compare this to a hidden single, where a number appears as a candidate in multiple cells in a row or box, but only one of those cells is actually in the right position. Hidden singles require a bit more work to spot.

For naked singles: look at the cell, eliminate, and the answer is naked — right in front of you.

How Often Do Naked Singles Appear?

More often than you think. In Easy and Medium difficulty puzzles, naked singles account for the majority of all moves. Some easy puzzles can be solved entirely with naked singles.

Even in Hard and Expert puzzles, naked singles appear — they're just harder to see because the board has fewer filled numbers to eliminate. Your eye has to scan more carefully.

Here's the thing most beginners miss: every time you place a number anywhere on the board, you may have just created a new naked single somewhere else. The solving process cascades. One solution creates another. This is why momentum in sudoku feels so good.

How to Train Yourself to Spot Them Faster

The difference between a beginner and an intermediate player is speed of pattern recognition. Here's how to build it:

• Scan systematically, not randomly. Go row by row, then column by column. Don't jump around the board hoping something jumps out.
• Focus on crowded rows and columns first. If a row already has 6 or 7 numbers filled in, the empty cells are prime naked single territory.
• Use pencil marks. Jot down all possible candidates for each empty cell. A cell with only one pencil mark is a naked single. This is how Sudoku Coach teaches you to spot them.
• After placing a number, immediately rescan its row, column, and box. You may have just created a cascade of new naked singles.

Naked Singles vs. Other Basic Strategies

Naked Single vs. Hidden Single

A naked single: "This cell can only contain one number."
A hidden single: "This number can only go in one cell in this row/column/box."
Same result, different angle of reasoning. Both are beginner-friendly — hidden singles just require scanning in reverse.

Naked Single vs. Naked Pair

A naked pair is the next level up: two cells in the same row, column, or box that together can only contain two specific numbers. You can't solve either cell individually, but you can eliminate those two numbers from everywhere else in the unit. Naked singles are the foundation; naked pairs are the next floor up.

The Mental Model That Changes Everything

Here's what really shifts when you truly understand naked singles: you stop thinking about sudoku as a guessing game and start thinking of it as a logic puzzle where answers are provable.

There's never a moment in sudoku where you should guess. Every correct answer has a reason. Naked singles are the simplest reason: "Every other number is already taken. This must be it."

Once that mental model is in place — that sudoku is elimination, not guessing — everything else follows. Hidden singles make sense. Naked pairs make sense. Even X-Wing and Swordfish start to make sense, because they're all just more sophisticated versions of the same idea: eliminate the impossible, and what remains is the answer.

Sherlock Holmes said it best: "When you have eliminated the impossible, whatever remains, however improbable, must be the truth." That's sudoku. That's a naked single. That's where it all starts.