Intermediate Sudoku Strategy
Locked Candidates
When all candidates for a digit in one box sit on the same row or column, remove that digit from the rest of the line.
Core concept
What it means
Locked candidates connect box logic with row and column logic. You do not place a digit immediately; you remove impossible candidates elsewhere.
The important part is that this is not a guess. A good Sudoku move should explain why at least one digit is forced or impossible. Locked Candidates gives you that explanation by connecting the three checks that define the puzzle: row, column, and 3x3 box.
Use this technique slowly at first. Name the container or region you are studying, list the legal candidates, and only then place a final digit or remove a note. That habit keeps the board readable when harder puzzles make several deductions interact at once.
Pattern triggers
When to use it
Look for Locked Candidates when direct scanning slows down and you need a concrete reason to place a digit or remove a candidate.
- Two or three candidate marks aligned inside one box.
- Rows that cross several busy boxes.
- Hard boards where singles are scarce.
Solving routine
Step-by-step method
Use this method slowly enough that every removal and placement can be checked against the grid.
- 1
Choose one 3x3 box and one missing digit.
- 2
Find every candidate position for that digit inside the box.
- 3
If all positions share one row or column, that digit is locked to the line inside the box.
- 4
Remove the digit from other cells on the same row or column outside the box.
Worked example
How it appears on a real board
If a box can place 3 only in row 6, then no other cell in row 6 outside that box can be 3.
After the deduction, update related cells immediately. A single placement changes its row, column, and 3x3 box, and stale notes are the fastest way to create mistakes.
Accuracy checks
Common mistakes
Most Sudoku errors come from moving before all three units agree. Before placing a final digit, check the row, the column, and the 3x3 box.
- Removing candidates before proving all box positions are aligned.
- Removing candidates inside the source box.
- Confusing a likely pattern with a locked one.
Practice checklist
Use it on your next board
Pick digit 5 and scan each box for aligned 5 candidates before placing any speculative digit.
- Name the row, column, or box you are studying.
- Remove candidates only when a visible rule explains the removal.
- After a placement, update every affected note before scanning elsewhere.
- Use hints only as a nudge, then continue with auditable logic.