Sudoku HintHow it works
When checking each cell, you often find multiple possible numbers. Because you cannot always decide immediately, writing possible numbers as notes helps you think through the puzzle. These are candidate notes.
As you write candidate notes in cells, if only one candidate remains, that cell is confirmed. If two or more remain, you use various techniques to reduce the candidates.
Once a cell's number is confirmed, you can remove that number from the candidate notes in the same row, column, and block. Removing candidates makes it easier to find the next confirmable cell.
Example
1
2
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4
5
6
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8
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Common mistakes
Candidate notes are possibilities, not confirmed numbers. Check the row, column and block and update notes after each placement.