Basic method
The simplest way to discover a number is when in a row, column or quadrant there is only one number left to place. In this case the missing number goes in the only empty cell.
Use after scanning, whenever a cell or a unit has only one option left.
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A naked single: this cell's row, column and box already contain eight different digits.
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Only the digit 4 is missing, so it is forced into the cell.
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A hidden single: 7 is blocked from every other cell of the box, so it belongs here even though the cell still shows other candidates.
As you can see, all numbers in the first row are placed except 7, so only this number can go in the empty cell. In the first column something similar happens with 5, as well as in the sixth quadrant with 1.
- Spotting a naked single but forgetting to remove that digit from the cell's peers.
- Overlooking hidden singles because the cell still shows several candidates.
Frequently asked questions
What is the difference between a naked and a hidden single?
A naked single is the last candidate in a cell; a hidden single is the last cell for a candidate in a unit. Both force a placement.