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Sudoku Pattern: Locked Candidates Patterns

These three cases are easy to recognize once all the candidates are revealed.

A. Candidates in a row that are constrained to one box

To find this pattern, looks at all the candidates in a row.  If any of the candidates in the row are all contained in (i.e. constrained to) a single box, then within that box this candidate can only be present in that row.  This allows us to eliminate that candidates from other rows within that box.

Look at the highlighted rows in the two examples given below.  In each row, try to find a candidate that exists only in one box. 

In the first example, on row 8, the candidate '1' exists only in the bottom middle box - hence it must be in the middle row within that box.  This allows us to eliminate '1' from the top row and bottom row within that box.  In the second example, on row 2, the candidate '5' exists only in the top middle box - hence it must be in the middle row within that box.  We can eliminate '5' from the top row within that box.

Sudoku Strategy: Locked Candidates   Sudoku Solve: Locked Candidates

B. Candidates in a column that are constrained to one box

To find this pattern, looks at all the candidates in a column.  If any of the candidates in the column are all contained in (i.e. constrained to) a single box, then within that box this candidate can only be present in that column.  This allows us to eliminate that candidates from other columns within that box.

Look at the highlighted columns in the two examples given below.  In each column, try to find a candidate that exists only in one box. 

In the first example, on column 7, the candidate '9' exists only in the middle right box - hence it must be in the left column within that box.  This allows us to eliminate '9' from the middle and right columns within that box.  In the second example, on column 9, the candidate '4' exists only in the top right box - hence it must be in the right column within that box.  We can eliminate '4' from the left and middle columns within that box.   (Notice that the same is true for candidate '3' but there is no '3' elsewhere in that box that can be eliminated.)

Sudoku Tips: Locked Candidates   Sudoku Hints: Locked Candidates

C. Candidates in a box that are constrained to one row or column

To find this pattern, looks at all the candidates in a box.  If any of the candidates in the box are all contained in (i.e. constrained to) a single row or single column within that box, then within that row or that column this candidate can only be present inside that box.  This allows us to eliminate that candidates from cells outside the box within that row or column.

Look at the highlighted boxes in the two examples given below.  In each box, try to find a candidate that exists only in one row or one column within that box. 

In the first example, in the middle-middle box, the candidate '5' exists only in the middle row of that box (row 6 of the puzzle) - hence it must be in the three cells of row 6 that are within that box.  This allows us to eliminate '5' from outside the box in row 6.  In the second example, in the middle left box, the candidate '6' exists only in the left column (column 1 of the puzzle) - hence it must be in the three cells of column 1 that are within that box.  We can eliminate '6' from outside the box in column 1.  (Notice that the same is true for candidate '9' but there is no '9' elsewhere in column 1 that can be eliminated.)


Sudoku Clues: Locked Candidates   Sudoku Techniques: Locked Candidates