Locked candidates

Locked candidates is the first technique that removes candidates instead of placing a digit. It has two forms. Pointing: in a box, all candidates for a digit lie in one row or column, so the digit is removed from the rest of that line. Claiming: in a row or column, all candidates for a digit lie in one box, so the digit is removed from the rest of that box. The Bare Sudoku hint labels the placement it unlocks with "Locked candidates".

When to use it

When singles run out in a Hard puzzle, this is the first thing to try. It needs complete notes for at least the digit you examine. Locked candidates are common: most Hard puzzles need them, and Expert puzzles use them between the bigger patterns.

How to spot it

For pointing, look at each box and each digit missing from it: if the candidate cells for that digit sit in a single row or column, check whether the same digit appears as a candidate further along that line outside the box. For claiming, look along a row or column: if the candidates for a digit are all inside one box, the rest of that box can be cleared. In the game, tap a digit key to highlight every candidate for it; a line of two or three highlighted notes inside one box is the pattern.

Example

Pointing

Pointing
c1c2c3c4c5c6c7c8c9
r136978137136742155678
r23463467785123679267347149678
r315283679673474967
r4716943582
r5923685174
r658412712727639
r72464671272567982453
r88346792372356714724557
r92337542378961

Bold digits are given, blue digits were placed while solving, small digits are candidates. Highlighted cells form the pattern, crossed-out candidates are removed, and a circled candidate is the digit to place.

  1. In box 1, digit 4 has only these candidate cells: r2c1, r2c2. All of them lie in row 2.
  2. Box 1 needs a 4, so the 4 of row 2 is inside box 1, whichever of those cells it lands in.
  3. Row 2 cannot hold a second 4, so remove 4 from the cells of row 2 outside the box: r2c7, r2c8.

Claiming

Claiming
c1c2c3c4c5c6c7c8c9
r146894678152369236937892368279
r23289169169741685
r356779412369837912361279
r4148147861279124578912958914583
r51489516134681362714
r6213478378179134578913961458149
r716915629312679457258
r871354812123596
r968936823892679267951234247
  1. In row 8, digit 2 can only go in r8c5, r8c6, and they all sit inside box 8.
  2. Row 8 must contain 2, so the 2 of box 8 is claimed by row 8. The other cells of box 8 cannot hold 2.
  3. Remove 2 from r7c5, r9c4, r9c5.

Common mistakes

In Bare Sudoku

Needed from level: Hard
In the game, the hint says: Locked candidates

Play Sudoku

Frequently asked questions

What are locked candidates in Sudoku?

Candidates for a digit that are confined to the intersection of a box and a line. Since the box, or the line, must contain the digit within that intersection, the digit can be removed from the rest of the line, or the rest of the box.

What is the difference between pointing and claiming?

Pointing starts from a box: the digit's candidates in the box point along one row or column, so the digit is removed from the rest of that line. Claiming starts from a row or column: the line's candidates for the digit are all in one box, so the line claims the digit and it is removed from the rest of the box.

Are locked candidates also called intersection removal?

Yes. Other names are box-line reduction for the claiming form and pointing pairs or pointing triples for the pointing form. Bare Sudoku uses one name for both.

Which level needs locked candidates?

Hard. A puzzle that needs locked candidates or subsets but nothing harder is rated Hard; Expert puzzles need a fish or a wing as well.

All techniques

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