How to use notes in sudoku

In Sudoku, notes are also called candidates or pencil marks. If a cell could still be a 2, 5, or 8, those numbers can be written as small notes inside the cell. They are not answers yet. They are possibilities.

Used well, notes make the puzzle easier to read. They help you see where a number can go, where it cannot go, and when a cell has only one possible answer left.

What notes mean in sudoku

A note is a small number written inside an empty Sudoku cell to show that the number might still be possible there.

For example, imagine an empty cell where the numbers 1, 3, 4, 6, 7, and 9 are already ruled out by the row, column, and 3×3 box. That leaves 2, 5, and 8 as possible candidates. You could write 2, 5, and 8 as notes in that cell.

Good notes are a way to record your logic. They show what is still possible, so you can look for what is forced.

Notes are not guessing

One common misunderstanding is that using notes means guessing. It does not.

Using notes means tracking what is still possible based on the current puzzle. Guessing means placing a number without being able to explain why it must be correct.

A candidate is only a possibility until you can prove it must be the answer.

In a valid Sudoku puzzle with one solution, the puzzle should be solvable through logic. Notes help you find that logic. They do not replace it.

When to start using notes

You do not always need notes from the first move.

In very easy Sudoku puzzles, many numbers may be obvious. You can often solve the first part of the puzzle by scanning rows, columns, and boxes. Adding notes too early can make the board look crowded before you need them.

A good beginner habit is to start using notes when the next move is no longer obvious. If you have scanned the puzzle and cannot quickly see a clear number to place, notes can help you move forward.

For medium and harder puzzles, notes become much more important. They make it easier to find singles, eliminate candidates, and spot patterns that are difficult to see in your head.

How to add notes step by step

To add useful notes, choose one empty cell and ask which numbers are still possible there.

1. Start with the row. If a number already appears in the same row, it cannot go in the cell.
2. Then check the column. If a number already appears in the same column, it cannot go in the cell.
3. Finally, check the 3×3 box. If a number already appears in the same box, it cannot go in the cell.

The numbers that remain are the candidates for that cell.

For example, if the row already contains 1, 4, and 9, the column contains 2 and 6, and the box contains 3 and 7, then those numbers are not possible in the cell. The only remaining candidates are 5 and 8. So you would write 5 and 8 as notes.

This simple process is the foundation of candidate-based Sudoku solving.

Notes must be kept updated

Notes are only useful if they match the current state of the puzzle.

Every time you place a number in a cell, that number is no longer possible in any other empty cell in the same row, column, or 3×3 box. If you place a 6, then 6 should be removed as a candidate from all related cells.

If you forget to remove old candidates, your notes can become misleading. You may spend time looking at possibilities that are no longer possible, or overlook a logical move because outdated notes make the board harder to read.

This is one of the reasons many players find Sudoku apps helpful. In MarApp Sudoku, when you place a number, candidates that are no longer possible in the same row, column, and 3×3 box are removed automatically from the notes.

Manual notes and auto-notes

Manual notes mean that you add and manage the candidates yourself. This is a good way to learn because it makes you look carefully at the row, column, and box for each cell.

Auto-notes fill in candidates for you. This can be useful when the puzzle is harder, the board is crowded, or you want to focus more on solving patterns than on maintaining notes.

Both approaches can be useful, but auto-notes should not become a substitute for understanding.

In MarApp Sudoku, manual notes are available while solving, and auto-notes are part of Premium.

How notes help you find logical moves

Once notes are in place, they help reveal moves that may not be obvious from the main numbers alone.

The simplest example is a cell with only one candidate. If a cell can only be 7, then 7 is the answer for that cell.

Notes can also show when a number only fits in one place within a unit (row, column, or 3×3 box). For example, a row may have several empty cells, but only one of them allows the number 4. In that case, 4 must go there.

This is why notes are so useful. They turn a difficult visual puzzle into a set of smaller logical questions.

Instead of asking “what number should I try here?”, you ask “what numbers are still possible, and which one is forced?”

As puzzles get harder, many Sudoku techniques are less about placing a number right away and more about eliminating candidates that basic techniques do not immediately rule out.

Related guide:
Learn Sudoku Techniques – MarApp Guide

Common mistakes when using notes

A common mistake is writing too many notes too early. If every empty cell is filled with candidates before you have made the obvious moves, the board can become difficult to read.

Another mistake is forgetting to remove old candidates. A note that was correct five moves ago may be wrong now. If your notes are not updated, they can hide the real logic of the puzzle.

It is also easy to use notes to justify a guess. For example, if a cell has candidates 3 and 8, choosing 3 just because it “feels right” is still guessing. The better question is: can you prove that 8 is impossible, or that 3 is required?

This is where Sudoku becomes more satisfying. The goal is not to try numbers and see what happens. The goal is to understand why a number must go in a specific place.

Related guide:
Beginner Sudoku – mistakes to avoid

When to hide notes

Notes are helpful, but they can also make the board feel busy.

Sometimes it is easier to hide notes for a moment and look only at the placed numbers. This can help you scan rows, columns, and boxes without distraction.

Then, when you need to analyze candidates again, you can show the notes.

This back-and-forth can be useful. A clean board helps you see the overall structure, while notes help you study the details.

Hiding and showing notes is not a practical option when you solve sudoku on paper. But some of the best sudoku apps, like MarApp Sudoku, include tools to hide/show notes, so you can easily switch between a cleaner board and a candidate-focused view while solving. In MarApp Sudoku this is a Premium tool.

Using highlighting together with notes

Highlighting tools in sudoku apps can make notes easier to understand.

If you select a number, highlighting can help you see where that number already appears and where it might still be relevant. This can make it easier to scan the board and avoid missing obvious eliminations.

Highlighting does not solve the puzzle for you. It simply makes important information easier to see.

Used together, notes and highlighting can reduce mental clutter. Notes show what is possible. Highlighting helps you see relationships across the board.

What to do when notes do not help

If you are stuck even with notes, it does not mean you need to guess.

First, check whether your notes are updated. Old candidates can make a puzzle look harder than it really is.

Then look for cells with only one candidate. After that, check each row, column, and 3×3 box to see whether a number has only one possible place in those units.

If there are no simple moves, the puzzle may require a more advanced logical pattern. This is normal, especially in harder Sudoku puzzles.

In MarApp Sudoku Premium, Surrender can help when you are stuck by showing the next logical step. This is different from simply guessing or revealing the entire solution. Used carefully, it can help you understand what kind of move you missed. With time, this may help improve your Sudoku solving skills.

A simple note-taking routine for beginners

A good routine makes notes easier to use.

1. Start by filling in any obvious numbers before adding many notes.
2. When the next move is not clear, begin adding candidates to selected areas of the board.
3. After placing a number, update the notes in the same row, column, and 3×3 box. Remove candidates that are no longer possible.
4. Then look for cells with only one candidate.
5. Next, look for numbers that appear as candidates in only one place within a unit.
6. If you get stuck, do not immediately guess. Re-check the notes, look for outdated candidates, and scan the board again.

Another useful tip is to start placing notes in units that only have two empty cells. If this doesn’t help you solve the puzzle, advance to placing notes in units with three empty cells, etc.

Often, the next logical move becomes visible once the notes are clean.

Should beginners use notes?

Yes, beginners should learn to use notes, but they do not need to use them for every single cell from the beginning.

Notes are most useful when they support your thinking. If they make the puzzle clearer, use them. If they make the board feel cluttered, use fewer notes or hide them temporarily.

The important thing is to understand what the notes mean. It is a candidate that remains possible based on the current puzzle.

Once you understand that, notes become one of the best tools for solving Sudoku logically.

Practise notes in MarApp Sudoku

The best way to get comfortable with notes is to use them while solving real puzzles.

MarApp Sudoku is built around logical solving, with manual notes, highlighting tools, hide/show notes, and optional auto-notes for Premium players.

You can start with easier puzzles, use notes when you need them, and gradually build stronger solving habits.