Placed a Number? Your Sudoku Notes Need One More Move.

You finally place a 5. Then you stare at the grid, waiting for the next breakthrough. It may already be there, hidden behind a tiny 5 you forgot to erase.

Pencil marks are a record of what could fit in each empty cell. When you place a digit, some of that record becomes outdated. For beginners learning to use notes, a short cleanup routine can reveal the next move without learning a harder technique.

Here is how to update those notes, which cells to leave alone, and what to check before trusting a newly revealed single.

A note is a possibility, not a promise

A small 5 in an empty cell means that 5 has not yet been ruled out there. It does not mean that the cell will eventually contain 5.

For basic notes, start with the digits from 1 to 9 and exclude anything already placed in the cell’s row, column, or 3×3 box. Further logical deductions can remove more candidates. Our solving techniques guide explains several of those deductions.

After you place a digit, that digit cannot appear in any other cell sharing its row, column, or box. Those related cells are often called its peers. Updating their notes keeps your record in step with the grid.

After a placement, check three places

Suppose you have proved that a cell must contain 5. Enter it, then:

Remove all pencil marks from the cell you just filled.
Scan its row and erase 5 from every other empty cell’s notes.
Do the same in its column.
Do the same in its 3×3 box.

Some cells belong to two of those groups. You only need to remove their 5 once.

You are removing 5, not all the notes in those cells. And you are checking the three groups that contain the new digit, not every 5 on the board.

If your app removes conflicting notes automatically, check the result before moving on. On paper, use the same row–column–box order each time so it is easier to remember what you have checked.

Watch a 5 reveal three next moves

We will label cells by row and column: r5c5 means row 5, column 5, counting from the top left. It is the center cell of the grid.

Assume that your earlier entries are correct, that you have proved r5c5 is 5, and that the notes below are complete for these example cells. These are selected cells illustrating the cleanup rule, not a full playable puzzle.

Cell	Notes before placing 5	Relationship to r5c5	Notes afterward
r5c2	2, 5	Same row	2
r1c5	5, 7	Same column	7
r4c6	1, 5	Same 3×3 box	1
r1c1	5, 9	Different row, column, and box	5, 9

The first three cells each lose 5. Because their lists were complete, the remaining candidate is forced: r5c2 is 2, r1c5 is 7, and r4c6 is 1. These are naked singles: empty cells with only one candidate left.

Notice r1c1. Its 5 survives because that cell shares none of the three groups with r5c5. There are nine 5s in a completed classic Sudoku, one in each row. Placing one 5 does not eliminate the digit everywhere else.

In your own puzzle, enter newly found singles one at a time and repeat the cleanup routine after each placement. A new entry can change the notes you were about to inspect next.

One remaining note is only useful if the list was complete

Perhaps you write down just the candidates you are currently investigating. That is a valid way to keep a reminder, but it is not a complete candidate list.

If you have written only 2 and 5 in a cell where 7 is also possible, deleting 5 leaves a visible 2. It does not prove that the cell contains 2. The unwritten 7 still matters.

Before treating a lone note as a forced move, ask:

Did I record every candidate for this cell?
Were any other candidates removed by valid deductions?
Are the placed digits that justify those removals correct?

If you are unsure, recheck the cell against its row, column, and box. Remember that a candidate can also have been ruled out by an earlier logical deduction, even if it does not conflict directly with a placed digit.

An empty candidate list means stop and check

Cleanup should not leave a genuinely empty cell with no possible digit in a consistent puzzle state. If it does, do not fill it with your best guess to keep going.

First check whether you missed a candidate when making the notes. Next, check whether you erased a digit from a cell that does not share the relevant row, column, or box. Finally, revisit the placement and the deductions leading to it. A wrong earlier entry can make later notes look impossible.

Undoing an entry also needs care. Candidates removed because of that entry may become possible again, but other valid constraints may still exclude them. Recheck the affected cells instead of restoring a digit everywhere automatically.

Try the routine on your next placement

Open today’s Sudoku and choose a move you can explain. After entering it, scan its row, column, and box for notes containing that digit. Then look at the cells whose notes changed.

Your aim is not to cover the board with tiny numbers. It is to keep a reliable record that helps you see the next deduction. Before searching for a new pattern, finish updating the consequences of the move you just made.