The case notes
Find a cell with one possible digit
A naked single is an empty cell with just one possible digit. Let us inspect the top-left cell. Watch a worked example, then try the idea in a puzzle.
Put it into practice
One cell · one possible digit
A naked single is an empty cell with just one possible digit. Let us inspect the top-left cell.
Row leaves 1, 2 or 5
The top row has three, four, six, seven, eight and nine. Its missing digits are one, two and five. Any of those could start here, so the row alone is not enough.
Column rules out 2
Look down its column. Two is already there. This cell cannot be two, so cross it off.
Box rules out 5
The top-left box already has a five. That removes five as well. Only one remains for our cell.
r1c1 = 1
Place one. There are still other gaps in the row, but this cell has a single candidate after its row, column and box agree.
Read the transcript
A naked single is an empty cell with just one possible digit. Let us inspect the top-left cell. The top row has three, four, six, seven, eight and nine. Its missing digits are one, two and five. Any of those could start here, so the row alone is not enough. Look down its column. Two is already there. This cell cannot be two, so cross it off. The top-left box already has a five. That removes five as well. Only one remains for our cell. Place one. There are still other gaps in the row, but this cell has a single candidate after its row, column and box agree.
