Locked Candidates

What Are Locked Candidates in Sudoku?

Locked candidates is a candidate elimination technique that exploits the intersection between a block and a row or column. Each intersection consists of exactly three cells. When a particular digit's candidates within a block or within a row/column are confined entirely to one of these three-cell intersections, the digit is said to be "locked" into that intersection. The technique comes in two flavors: - Pointing (Type 1): The logic flows from the block outward into the row or column. - Claiming (Type 2 / Box-Line Reduction): The logic flows from the row or column inward into the block. Both rely on the same underlying principle.

The Core Principle Behind Locked Candidates

A block intersects a row in exactly three cells. Consider digit 5 and a particular block. If digit 5's remaining candidates within that block all happen to fall in the same row, then digit 5 will be placed in one of those intersection cells. That means digit 5 cannot appear anywhere else in that row. You can safely remove digit 5 from every other cell in the row outside the block. The reverse works too. If digit 5's candidates within a particular row all fall inside a single block, then digit 5 must occupy one of those intersection cells. You can eliminate it from the block's other cells.

Pointing: Locked Candidates Type 1

Pointing occurs when all candidates for a digit within a block are aligned along a single row or a single column. The candidates "point" outward from the block along that line. You can eliminate that digit from every other cell in the row or column that lies outside the block. The pattern is named pointing pair when exactly two candidate cells exist in the intersection, and pointing triple when three candidate cells exist. The logic is identical regardless of count.

Claiming: Locked Candidates Type 2 (Box/Line Reduction)

Claiming is the reverse of pointing. It occurs when all candidates for a digit within a row or column are confined to a single block. The row or column "claims" the digit for that block, meaning the digit cannot appear elsewhere in the block. You eliminate the digit from every other cell in the block that is outside the row or column. This pattern is also widely known as box/line reduction or line/box reduction.

How to Find Locked Candidates in a Sudoku Puzzle

Strategy 1: Scan by Block. For each block, examine each unplaced digit. Identify all cells where the digit is a candidate. Check whether all those cells share the same row or column. If yes, eliminate from the rest of that line outside the block. Strategy 2: Scan by Row or Column. For each line, examine each unplaced digit. Check whether all candidate cells fall within the same block. If yes, eliminate from the rest of that block outside the line. Practical Tips: - Start with digits that have few remaining candidates. - Look for blocks that are mostly solved. - Combine with pencilmark notation. - Check after every elimination.

Why Locked Candidates Matter for Sudoku Solving

Locked candidates occupy a critical position in the hierarchy of Sudoku solving techniques. They bridge the gap between basic strategies and advanced patterns. Naked singles and hidden singles will carry you through most easy puzzles. Pairs extend your reach into medium territory. But many medium and hard puzzles contain bottlenecks where singles and pairs are not enough. Locked candidates frequently provide the breakthrough elimination. Understanding locked candidates also builds intuition essential for more complex techniques like fish patterns and Almost Locked Sets. Locked candidates patterns appear with high frequency in puzzles rated medium or harder. You will use this technique constantly.

Difficulty Level and Classification

Locked candidates is classified at Level 3 (Easy), alongside hidden pairs and naked pairs. If you are learning Sudoku techniques in order, locked candidates should be the third or fourth strategy you add: 1. Full house and naked/hidden singles 2. Naked pairs and hidden pairs 3. Locked candidates (pointing and claiming) From here, you are well prepared to tackle naked triples, X-Wings, and beyond.

Common Terminology and Alternate Names

Locked candidates: The general technique covering both sub-types Pointing: Type 1 -- block constrains the row/column Pointing pair: Pointing with exactly 2 candidate cells Pointing triple: Pointing with exactly 3 candidate cells Claiming: Type 2 -- row/column constrains the block Box/Line Reduction: Same as claiming Line/Box Reduction: Same as claiming (reversed word order) Intersection removal: General term for either sub-type Regardless of the name, the underlying logic is always the same: candidates confined to an intersection allow eliminations in the house on the other side.

Summary

Locked candidates is a foundational Sudoku technique that every solver should learn early. It uses the intersection between a block and a row or column to eliminate candidates. When a digit's possibilities within a block are locked into a single row or column (pointing), you eliminate from the rest of that line. When a digit's possibilities within a row or column are locked into a single block (claiming), you eliminate from the rest of that block. The technique is classified as Level 3 (Easy), appears frequently in medium-difficulty puzzles, and serves as a stepping stone to advanced strategies.