Sudoku · EN
Sudoku Locked Candidates: Worked Example
Locked candidates connect a box with a row or column. If every possible place for one digit in a box lies on the same line, remove that digit from the rest of the line outside the box.
Solver-verified example
Starting grid; dots are empty cells.
Candidates before the move
- R1C7 = {2, 4, 5, 7}
- R1C8 = {1, 2, 4, 7}
Work through this deduction
- Find the candidate 4 in the upper-right 3×3 box: only row 1, columns 7 and 8 can hold it.
- Both possible cells lie in row 1. Whichever one takes 4, that row already gets its 4 inside this box.
- Remove 4 from row 1, columns 4, 5 and 6, which are outside that box.
- Keep 4 as a possibility in the two highlighted box cells. This deduction does not decide which one is 4.
Eliminate: R1C4 ≠ 4; R1C5 ≠ 4; R1C6 ≠ 4.
Independent verification found one completion. Forcing each displayed eliminated candidate separately produced zero completions. The grid and candidate pattern explain the move; the final answer is a check.
Verified completion
Why this deduction works
The box must contain the digit once. Whichever highlighted cell takes it, that line already contains the digit. This excludes other positions on the line without deciding the position inside the box. Check every candidate in the box: one off the line would invalidate the pattern.
Sources and background
Frequently asked questions
Does this move finish the puzzle?
No. It establishes only the stated placement or eliminations. Update affected candidates and continue with another supported deduction.
Are incomplete notes sufficient?
No. Candidates must include every digit still permitted by the row, column and box. Missing a legal candidate can create a false pattern.
