Locked Candidates

Box-Line Interactions for Powerful Eliminations

By Minimal Sudoku Team•Last updated: January 28, 2025

Locked Candidates is a family of techniques that exploit the intersection between 3×3 boxes and lines (rows/columns). When candidates are "locked" to specific cells at these intersections, eliminations follow.

⚡Quick Summary

Two types: Pointing Pairs/Triples and Claiming Pairs/Triples
Core idea: When candidates in one unit are confined to an intersection with another unit
Result: Eliminate candidates from the non-intersecting part of the other unit
Difficulty: Intermediate — bridge between basic and advanced techniques

The Concept

Every 3×3 box intersects with exactly 3 rows and 3 columns. At each intersection, there are 3 cells. Locked Candidates techniques use these intersections to create eliminations.

🔒 Locked Candidates Principle

When a candidate within one unit (box or line) is confined to cells that also belong to another unit, that candidate is "locked" — and can be eliminated from the non-shared cells of the second unit.

The Two Types

👉 Pointing Pairs/Triples

Candidate in a box is confined to one row/column.

→ Eliminate from the rest of that row/column.

📍 Claiming Pairs/Triples

Candidate on a row/column is confined to one box.

→ Eliminate from the rest of that box.

These are mirror techniques — same logic, different starting points.

Type 1: Pointing Pairs & Triples

Starting point: A 3×3 box

If a candidate within a box can only appear in cells that are all on the same row (or column), the candidate "points" outward. Since the box will definitely place that candidate on that line, no other cell on the line can have it.

Box 1:                    Row 2 continues:
┌─────────────────┐       
│  .  │  .  │  .  │       
│─────│─────│─────│       
│ [5] │ [5] │  .  │  →→→  │  ✗  │  ✗  │  ✗  │  ✗  │  ✗  │  ✗  │
│─────│─────│─────│       
│  .  │  .  │  .  │       (5 eliminated from rest of row 2)
└─────────────────┘

Intermediate

Pointing Pairs & Triples

Complete guide with step-by-step examples and detection tips.

Type 2: Claiming Pairs & Triples

Starting point: A row or column

If a candidate on a row (or column) can only appear in cells that are all in the same box, the line "claims" those cells for that candidate. Since the line will definitely place that candidate in this box, no other cell in the box can have it.

Row 2: │ . │ . │ . │ [7] │ [7] │ . │ . │ . │ . │
                       ↓    ↓
                    These are in Box 2
                    
Box 2:
┌─────────────────┐
│ [7] │ [7] │  ✗  │  ← Row 2's 7 must be here
│─────│─────│─────│
│  ✗  │  ✗  │  ✗  │  ← Eliminate 7 from other cells in box
│─────│─────│─────│
│  ✗  │  ✗  │  ✗  │
└─────────────────┘

Intermediate

Claiming Pairs & Triples

The mirror technique — when lines claim candidates in a box.

When to Look for Locked Candidates

After Placing Numbers

Each placement changes candidate distributions. Re-check the affected box and lines for new Locked Candidate patterns.

Nearly-Complete Units

Boxes or lines with 6-7 filled cells often have candidates confined to small areas — prime territory for Locked Candidates.

Snyder Notation Reveals Them

Snyder Notation marks candidates when they're limited to 2 cells in a box. If both marks are on the same row or column, you've found a Pointing Pair!