Locked candidates

Hard

Locked candidates are about a digit that is stuck: inside a box, all its places lie on one row or column, or along a line, all its places lie in one box. It’s the most common step in Hard puzzles, and the first that needs notes.

Pointing: from a box to a line

If every place for a digit inside a box lies on the same row (or column), the box’s copy of that digit is somewhere on that line. The line can’t have two, so no other cell of the line, outside the box, can hold the digit.

22222222222222222222222222222226519159696713476192957611597698719451816941A2B3C4D5E6F7G8H9I
Only the notes for 2 are shown. In the bottom-middle box, a 2 can only go in I4 and I6, both on row I. One of them will be the 2, so no other cell of row I can be: the 2s in I1 and I2 go.

Claiming: from a line to a box

It works the other way round too. If every place for a digit along a row (or column) lies inside the same box, that line’s copy is in the box, so the box’s other cells can’t hold the digit.

11111111111112394896591348267594238119537184921A2B3C4D5E6F7G8H9I
Only the notes for 1 are shown. On column 2, a 1 can only go in D2 and E2, both in the middle-left box. So the 1 of the middle-left box has to be one of them, and the 1s in D3 and E3 go.

How to spot them

  • Take one digit at a time and look at its notes box by box: two or three notes in a straight line are what you’re after.
  • The notes it removes often leave a hidden or naked single right away, so look for one before moving on.
  • In Sudoku Knockout’s hints, this shows up as “Locked candidates”: the hint tells you when a puzzle needs it.

Try it on a real puzzle

Hard puzzles in Sudoku Knockout need techniques like this one, and never a guess. When you’re stuck, a hint shows you the step.

Play a Hard puzzle