How to Use Notes (Pencil Marks) in Sudoku

Notes (also called pencil marks or candidates) are the small digits you write in an empty cell to record which numbers could still go there. Use them when scanning stops finding answers: list the possible digits for each empty cell, erase them as the board fills in, and the solution often shows up as a cell with only one note left.

Notes don't solve anything by themselves. They hold information you would otherwise have to keep in your head, so you can spot patterns that are invisible on a bare grid. If you're new to the game, start with the rules and come back here.

What a note means

A note says "this digit is not ruled out yet." It is not a guess. A digit belongs in a cell's notes when it doesn't already appear in that cell's row, column or 3×3 box.

Here is how to work out one cell. Say you're looking at row 1, column 5:

• Row 1 already has 1, 4 and 9.
• Column 5 already has 2 and 7.
• The top-middle box already has 3 and 5.

That rules out 1, 2, 3, 4, 5, 7 and 9. Only 6 and 8 are left, so you write a small 6 and a small 8 in the cell. Later, if a 6 is placed anywhere in column 5, you erase the 6 note and the cell is an 8.

When to start taking notes

On Easy puzzles you can usually finish by scanning alone, and filling in notes would only slow you down. On Medium and harder puzzles there comes a point where no cell has an obvious answer. That's when notes pay off.

A good habit: scan first for naked and hidden singles until you run dry, then start noting. You'll have fewer empty cells to fill in, and fewer candidates in each.

Two ways to take notes

Full candidates. Write every possible digit in every empty cell. It takes a while, but nothing is hidden: every technique, from pairs to x-wings, works directly on a full set of candidates. This is the style most digital players use.

Sparse notes. Only mark a digit in a box when it has exactly two possible cells there (some players allow three). This is often called Snyder notation. It's faster on paper and keeps cells uncluttered, but it misses some patterns, so most players switch to full candidates when a puzzle gets hard.

Neither is wrong. If you're learning, full candidates are easier to trust because you can check any cell at a glance.

Keep your notes honest

Notes are only useful if they are correct. Two rules keep them that way:

1. Every time you place a digit, remove it from the notes of its row, column and box. Forgetting this is the most common reason a note-based solve goes wrong.
2. Only erase a note when you can say why. "It felt wrong" is not a reason. A placed digit, a pair or a pointing pair is.

Reading your notes

Once the notes are in, look for these patterns, roughly in this order:

• A cell with one note. That's a naked single. Place it.
• A digit that appears in only one cell's notes within a row, column or box. That's a hidden single. It goes there, even if the cell has other notes.
• Candidates for a digit that line up inside a box. A pointing pair lets you clear that digit from the rest of the line.
• Two cells in the same unit with the same two notes. If two cells in a box both have only 2 and 6, those two cells take the 2 and the 6 between them. Erase 2 and 6 from every other cell in that box. That's a naked pair, and hidden pairs work the same way in reverse.

Every elimination can create a new single, so after each one, scan the cells you just touched.

How notes work in Kodiak Sudoku

• Notes mode. Tap the Notes button (the pencil) or press N on a keyboard. While it's on, typing or tapping a digit adds or removes that note on the selected cell instead of placing it.
• One note without switching modes. Hold a number pad button for a moment to toggle that digit as a note on the selected cell while you stay in normal mode.
• Automatic cleanup. Placing a digit clears the notes in that cell and removes that digit from the notes of every cell in the same row, column and box. You still need to make technique-based eliminations yourself.
• Auto-notes. The Auto-notes button fills every empty cell with its candidates, based on the digits already placed. Press it again when the notes still match and it clears them. You can hide the button in Settings if you'd rather do it all by hand.
• Speed Notes. Shift+N fills candidates for the selected cell, Ctrl+Shift+N fills every empty cell, and holding the Notes button does the same. These add to your existing notes rather than replacing them. Speed Notes can be turned off in Settings too.
• Digit highlighting. Select a cell with a digit in it and that digit lights up in the notes of every empty cell. It's the quickest way to see where one digit can still go.
• Undo covers notes. Note changes, including a bulk fill, can be undone like any other move.

Common mistakes

• Auto-filling in the middle of a solve. Filled candidates only account for placed digits. Auto-notes replaces your notes outright, and Speed Notes adds candidates back in, so either one can bring back digits you had already eliminated with a pair or an x-wing. Fill once near the start, then maintain the notes by hand.
• Placing a note as a digit. It's easy to forget which mode you're in. If a big digit appears where you meant a small one, undo it.
• Treating notes as guesses. A note never means "probably." If you find yourself noting digits you think are likely, stop and recount the row, column and box.

Practice

Pick a Medium puzzle, solve what you can by scanning, then switch to notes for the rest. When you get stuck, the Technique Journey on the learn page walks through what to look for in your notes, one technique at a time.

Or take on today's daily puzzle and see how much faster it goes once your notes are clean.