1️⃣ Naked Single

Naked Single is the cornerstone of every Sudoku player's toolkit. It is the simplest and most frequently used technique — if you know nothing else, you can still solve many Easy and even some Medium puzzles using it alone. The idea is direct: if only one digit can legally go into a cell, that digit must go there.

What is a Naked Single?

A Naked Single arises when an empty cell has exactly one remaining candidate — all other eight digits (1–9) are already present in cells sharing the same row, column, or 3×3 box. Because each digit must appear exactly once in each unit, that last remaining digit has nowhere else to go: it belongs in this cell.

The word naked means the single candidate is immediately visible — you do not need to scan other cells or think about complex patterns. Everything you need is right there in the cell's own row, column, and box.

Step-by-step guide

1. Pick any empty cell. You can scan row by row, or start in areas of the board that are most filled in.
2. Collect the digits already placed. Look at the cell's row (8 other cells), column (8 other cells), and 3×3 box (8 other cells). Together these form the cell's peer set.
3. Count what is missing. Note which digits 1–9 do not appear in those peers. If exactly one digit is absent, that is the Naked Single — write it in the cell.
4. Cascade the discovery. Placing a digit often creates new Naked Singles nearby. After every placement, re-examine the row, column, and box you just modified.
5. Repeat until stuck. When no more Naked Singles exist, move on to a more advanced technique.

Row contains 1 2 3 4 6 7 8 9 — only 5 is absent → the amber cell is a Naked Single

Why does it work?

Sudoku's fundamental rule states that each digit 1–9 must appear exactly once in every row, column, and 3×3 box. If eight of the nine digits are already placed in the units containing a cell, only one digit is legally allowed. There is no ambiguity — the solver does not guess. The logic is certain and immediate.

This is why Naked Singles are so powerful: every placement is 100% guaranteed correct. You never risk creating an inconsistency.

Three flavours of Naked Single

A Naked Single can be triggered by any one of the three units — or a combination of them:

• Row single. All eight other digits already appear somewhere in the same row. The ninth digit goes here.
• Column single. All eight other digits appear somewhere in the same column.
• Box single. All eight other digits appear somewhere in the same 3×3 box. Box singles are often the easiest to spot visually because the box is a compact block.
• Combined. More commonly, the peer set uses a mixture — some digits are blocked by the row, others by the column, others by the box. The result is the same: only one digit survives.

Cascading placements

One of the most satisfying aspects of Naked Singles is that they chain. Placing digit 5 in a cell removes 5 as a candidate from every peer of that cell — up to 20 other cells. This sometimes reduces another cell in the same row, column, or box to a single candidate, creating a new Naked Single. That placement in turn creates more. A well-constructed Easy puzzle can be solved in one long unbroken cascade, each Naked Single leading to the next.

Always re-examine the row, column, and box immediately after placing a digit. This disciplined habit catches new Naked Singles before you move on and prevents unnecessary re-scanning of the entire board.

Recognising it quickly

Beginners often check each unit separately. With practice you start scanning the entire 20-cell peer set in one visual sweep. A useful shortcut: look first at cells that sit in dense areas of the board — rows and columns already heavily filled. If a row has 7 givens and a column has 6, the cell at their intersection might need only a glance to confirm its single candidate.

In SudokuStreak, activate pencil mode to write candidates into each empty cell. When a cell shows only one pencil mark, it is a Naked Single — tap it and press the digit to place it.

Relationship to other techniques

Naked Single sits at the very bottom of the technique hierarchy. If you can solve a cell with a Naked Single, always do so — it is faster and safer than any other approach. Save harder techniques for when simpler ones fail.

• After exhausting all Naked Singles, look for Hidden Singles.
• When Hidden Singles are also exhausted, consider Naked Pairs and more advanced techniques.
• Naked Singles appear after every application of every other technique — always check for them first after any board change.

Common mistakes

• Forgetting one of the three units. Always check row, column, AND box. Checking only one or two can lead to a false Naked Single and a wrong placement.
• Not cascading. After placing a digit, forgetting to check nearby cells for new Naked Singles leads to unnecessary backtracking later.
• Skipping to advanced techniques too soon. On Hard and Expert puzzles, Naked Singles still appear frequently — especially after each elimination from a Naked Pair or X-Wing. Always check for them before escalating.