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Simply click on any empty cell and select a number from 1-9. Use logic to determine which numbers can go in each cell based on Sudoku rules - each row, column, and 3x3 box must contain all numbers 1-9 exactly once.

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What Are Locked Candidates in Sudoku? The Complete Guide for Intermediate Players

Introduction

Locked Candidates are one of the first "intermediate-level" techniques that players encounter when moving from Medium to Hard puzzles.

This method helps eliminate candidates and unlock cells that previously looked unsolvable.

If you want to level up your Sudoku skills, learning Locked Candidates is essential. This guide explains both types of Locked Candidates with clear examples and step-by-step instructions.

Key Points

Understanding these fundamentals helps you master Locked Candidates:

Locked Candidates help eliminate conflicting numbers: Powerful elimination technique
Two forms: Pointing (Type 1) and Claiming (Type 2): Different elimination directions
Extremely useful in Medium & Hard puzzles: Essential for intermediate solving
One of the fastest ways to clear pencil marks efficiently: Significant candidate reduction
Works perfectly with Notes mode on SudokuGames.org: Notes reveal these patterns

What Are Locked Candidates?

Locked Candidates occur when:

A number is restricted to a single row or column inside a box, or
A number is restricted to a single box inside a row or column.

When this happens, you can eliminate the number from the rest of that row/column/box.

The logic is minimal but powerful. Locked Candidates create large-scale eliminations from simple restrictions.

How It Works (Step-by-Step)

Mastering Locked Candidates requires systematic detection:

Step 1: Enable Pencil Marks

Turn on Notes Mode to reveal candidate clustering patterns. Notes are essential for spotting Locked Candidates because they show where numbers are restricted.

Step 2: Examine Each 3×3 Box

Identify numbers limited to one row/column (Pointing). Check if a candidate appears only in cells within a single row or column of the box.

Step 3: Examine Each Row & Column

Identify numbers that appear only in one box (Claiming). Check if a candidate in a row or column is restricted to a single box.

Step 4: Eliminate Candidates

Remove the number from conflicting cells. Pointing eliminates outside the box; Claiming eliminates inside the box.

Step 5: Re-scan

Often a cascaded chain of singles appears. Locked Candidates frequently reveal immediate progress through eliminations.

Two Types of Locked Candidates

There are only two kinds:

Type 1: Pointing (Pointing Pair / Pointing Triple)

A number appears only in one row or column inside a 3×3 box.

Example:

In Box 4 (middle-left box), the candidate 7 appears only in the top row of that box:

7 . .

. . .

Therefore, 7 must be placed somewhere in that row within the box

You can eliminate all other "7" candidates in that row in other boxes

Type 2: Claiming (Claiming Pair / Claiming Triple)

A number appears only in one box inside a row or column.

Example:

Row 5 has candidate "3" only in Box 6:

. 3 .

Therefore, 3 must be placed inside that box

You can eliminate "3" from all non-row cells inside that box

Why Locked Candidates Matter

Locked Candidates are powerful because they:

reduce cluttered notes — large-scale candidate elimination
reveal Naked Singles & Hidden Singles — eliminations create immediate placements
simplify complex grids — cleaner candidate grids
accelerate solving in Hard puzzles — essential for progression
reduce branching significantly — fewer possibilities to consider

They are one of the fastest "efficiency" techniques in modern Sudoku solving.

How to Identify Locked Candidates (Step-by-step)

Detection Process

Step 1 — Enable pencil marks

This reveals candidate clustering patterns.

Step 2 — Examine each 3×3 box

Identify numbers limited to one row/column (Pointing).

Step 3 — Examine each row & column

Identify numbers that appear only in one box (Claiming).

Step 4 — Eliminate candidates

Remove the number from conflicting cells.

Step 5 — Re-scan

Often a cascaded chain of singles appears after elimination.

Examples

Here are practical examples demonstrating Locked Candidates:

Example A — Pointing 9s

In a box, all candidate 9s are in column 2 → eliminate 9s from column 2 in other boxes. The pointing restriction allows elimination across the entire column outside the box.

Result: Large-scale elimination reveals immediate progress.

Example B — Claiming 4s

Row 3 has 4 only in Box 1 → eliminate 4s from Box 1 except in that row. The claiming restriction clears candidates from other cells in the box.

Result: Elimination creates space for other numbers in the box.

Example C — Triggering a Naked Single

After eliminating candidates through Locked Candidates, a cell in Box 7 becomes {6}, revealing a Naked Single. Eliminations cascade into immediate placements.

Result: Locked Candidates often trigger chains of placements.

Player Tips

Use these strategies to master Locked Candidates:

Use Pointing first, then Claiming — systematic approach
Apply Locked Candidates before heavy techniques like X-Wing — efficient order
Ideal for Medium & Hard puzzles — most effective at these levels
Works best with full pencil marking — complete candidate visibility
SudokuGames.org's Notes mode makes patterns easier to spot — leverage tools

Summary

Locked Candidates are one of the strongest intermediate techniques in Sudoku.

They improve efficiency, reduce clutter, and unlock Hard puzzle logic. Learning Type 1 (Pointing) and Type 2 (Claiming) will significantly speed up your solving progress.

Mastering Locked Candidates creates a foundation for learning advanced techniques. This intermediate skill bridges basic and advanced solving methods.

❓ FAQ

Q1: Is this technique required for Hard puzzles?

Yes. It appears frequently and is crucial for puzzle progression. Hard puzzles often require multiple Locked Candidate eliminations.

Q2: Is it harder than Hidden Singles?

Slightly, but the logic is still beginner-friendly. Once understood, Locked Candidates become intuitive and powerful.

Q3: Does it require full pencil marks?

Yes. Locked Candidates rely on candidate distribution. Without notes showing where candidates appear, detection is nearly impossible.

Q4: Can this technique solve puzzles alone?

Not fully—you'll also need Pairs, Triples, X-Wing, etc. Locked Candidates are one essential tool in a complete solving toolkit.

Q5: Does SudokuGames.org support note-taking?

Yes, with a clean and intuitive Notes mode ideal for spotting these patterns. Notes Mode makes Locked Candidates much easier to identify.

Q6: What's the difference between Pointing and Claiming?

Pointing restricts a number within a box to one row/column, eliminating outside. Claiming restricts a number in a row/column to one box, eliminating inside the box elsewhere.

Q7: Do Locked Candidates appear in Easy puzzles?

Rarely. They're most common in Medium and Hard puzzles where candidate density creates restriction patterns.

Q8: How often do I need to use this technique?

In Medium puzzles, Locked Candidates appear frequently. Hard puzzles often require multiple applications throughout solving.

Ready to master Locked Candidates? Start practicing with Notes Mode enabled today!

Play Hard Sudoku on SudokuGames.org | Learn More Strategies

Explore our technique guides to master intermediate strategies, practice with Hard Sudoku puzzles, and improve your solving skills!