I used a step-by-step Sudoku solver for a puzzle in which I was stuck.
However I don't understand how the next number (6) was found.

Sudoku puzzle where I was stuck:

sudoku stuck

Sudoku puzzle solved by one step forward:

sudoku one step forward

The solver says the only candidate for cell (R4, C5) is a 6. Why couldn't it be in cell (R6, C5)? I need a clear explanation; I'm not a Sudoku expert.


3 Answers 3


R6C5 doesn't have anything ruling out it being a 6... but R4C5 could only be a 6. There are no other options for R4C5: placing a 1, 2, 3, 4, 5, 7, 8, or 9 would break the rules. You know you have to fill a box with some number, and that is the only one left.

This is one of two basic Sudoku techniques, the "naked single" -- when a cell only has one option out of the 9 allowed digits, you can fill that number in. The other basic technique is the "hidden single" -- when there's only one place a certain digit can go in a row (or a column, or a 3×3 box). The confusion here seems to be coming from you assuming this is a hidden single, but in reality it is a naked single.

  • $\begingroup$ I feel so dumb right now, this looks obvious. Thank you very much $\endgroup$
    – Sudokunoob
    Commented Mar 20, 2021 at 15:51
  • 4
    $\begingroup$ @Sudokunoob Lots of things look obvious when you see them written out like this! Learning to find these "obvious" things is just part of the learning process. $\endgroup$
    – Deusovi
    Commented Mar 20, 2021 at 15:53

The only possible number that could go in cell (R4, C5) is 6, as shown by the the grayed out numbers: 3 and 5 are eliminated by the row, and the rest are eliminated by the column.


Nice to see it describes this way. The column C5 will only allow one of the 3,5,6 values in the available cells. R4 has the 3 and 5, which removes the 3 and 5 from the candidates. This forces the 6 onto that position. Refered to as the naked value.

  • $\begingroup$ Welcome to Puzzling! While this is correct, there are already two answers saying the exact same thing which were posted a while ago. Unless you have a substantial improvement on a previous answer, please don't post duplicates. $\endgroup$
    – bobble
    Commented Mar 21, 2021 at 17:22

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