--- title: "Sudoku Notes: When and How to Use Them | Sudoku Flow" description: "Sudoku notes track the digits a square can still hold. When you need them, two ways to write them, and a real puzzle where notes find the move." url: https://sudokuflow.com/guides/sudoku-notes-pencil-marks/ language: en last_modified: 2026-11-06 --- [Sudoku guides](https://sudokuflow.com/guides/) # Sudoku Notes: When and How to Use Pencil Marks Sudoku notes track the digits a square can still hold. When you need them, two ways to write them, and a real puzzle where notes find the move. Updated 2026-11-06 ## Quick answer Notes, also called pencil marks or candidates, are the small digits you write in a square to keep track of what could still go there. On Easy and Medium you can do without them: every Easy and Medium puzzle in our app can be finished with singles alone. From Hard on, the key moves work by ruling candidates out, and notes are how you see them. - Notes are the digits that could still go in a square. - Easy and Medium puzzles in our app need no notes. - Every Hard puzzle in our app needs at least one step that rules out candidates. - Two common styles: every candidate in every square, or marks only where a digit has two places left in a box. - Update your notes each time you place a digit. ## When to start taking notes When no square has a single digit left and no digit has a single place left in a row, column or box, it is time to write notes. In our app that never happens on Easy or Medium. On Hard it always happens at some point: every Hard puzzle needs at least one step that removes candidates rather than placing a digit. ## Two ways to write notes Full notes: in every empty square, write each digit that could still go there, small, always in the same spot (1 top left, 9 bottom right). It takes a while, but nothing is missed. Corner marks: write a digit small in the corners of a square only when that digit has two places left in a box. It is quicker, and the marks point straight at useful pairs. Our app has both: its Classic note style puts each digit in its own spot, and the Corner + Center style adds corner marks, with centre notes for what a square can hold. ## Notes at work on a real puzzle This is puzzle 1 on Hard sheet 1 of our printable Sudoku, after every single has been placed: the digits in blue were filled in, and now no square has one digit left and no digit has one place left. With full notes written in, a pattern shows. In one box, the 8 can only go in two squares, row 1, column 7 and row 2, column 7, marked on the board, and both are in column 7. So the 8 of that line is in this box, and it cannot go anywhere else in column 7. The note 8 comes out of row 7, column 7, which leaves that square with one digit: 7. ```text 3 6 4 | {289} 1 {258} | {589} {59} 7 1 2 5 | {689} {68} 7 | {389} {369} 4 8 9 7 | 3 {46} {456} | 1 2 {56} ---------------------+----------------------+--------------------- {45} {345} 2 | {146} 7 {146} | {359} 8 {359} 6 1 8 | 5 9 3 | 4 7 2 {457} {3457} 9 | {248} {48} {248} | 6 {35} 1 ---------------------+----------------------+--------------------- 2 {47} 3 | {468} 5 9 | {78} 1 {68} {459} {45} 1 | 7 3 {468} | 2 {4569} {5689} {4579} 8 6 | {14} 2 {14} | {3579} {3459} {359} ``` Struck-out notes (ruled out in this picture): 8 in r7c7. *Hard, sheet 1, puzzle 1, with full notes: the struck-out note is the one the marked squares rule out.* [Pointing Pair →](https://sudokuflow.com/lessons/pointingPair/) ## Keeping notes tidy - Each time you place a digit, take it out of the notes in its row, its column and its box. - Check your notes after a break: one note that should have gone can hide the next move. - On paper, write notes small and light, so they are easy to rub out. - If the notes get messy, rewrite them for one box at a time. [Stuck on a Hard or Expert Sudoku? The Technique You Need Next →](https://sudokuflow.com/guides/stuck-on-hard-sudoku/) ## Practice Every Find the Mistake puzzle on this site has exactly one wrong digit, and you can play each board right on its page. [Play the puzzles →](https://sudokuflow.com/puzzles/) ## Sudoku guides - [How to Find a Mistake in Sudoku](https://sudokuflow.com/guides/find-a-mistake/) - [7 Common Sudoku Mistakes and How to Avoid Them](https://sudokuflow.com/guides/common-mistakes/) - [Why Won't My Sudoku Solve? A Checklist](https://sudokuflow.com/guides/sudoku-wont-solve/) - [Naked Single vs Hidden Single](https://sudokuflow.com/guides/naked-single-vs-hidden-single/) - [Pointing Pair vs Box/Line Reduction](https://sudokuflow.com/guides/pointing-pair-vs-box-line-reduction/) - [X-Wing vs Swordfish](https://sudokuflow.com/guides/x-wing-vs-swordfish/) - [How to Start a Sudoku: The First Five Minutes](https://sudokuflow.com/guides/how-to-start-sudoku/) - [Stuck on a Hard or Expert Sudoku? The Technique You Need Next](https://sudokuflow.com/guides/stuck-on-hard-sudoku/) - [Sudoku Tips That Actually Work (and Is There a Formula?)](https://sudokuflow.com/guides/sudoku-tips/) - [How to Teach a Child Sudoku: 4×4, Then 6×6, Then 9×9](https://sudokuflow.com/guides/teach-kids-sudoku/) - [Can Every Sudoku Be Solved Without Guessing?](https://sudokuflow.com/guides/solve-without-guessing/) - [Which Sudoku Techniques Do You Actually Need? 60,000 Puzzles Counted](https://sudokuflow.com/guides/sudoku-techniques-you-need/) - [How to Solve Sudoku Faster](https://sudokuflow.com/guides/solve-sudoku-faster/) - [Do the Diagonals Count in Sudoku? Diagonal Sudoku (Sudoku X) Rules](https://sudokuflow.com/guides/diagonal-sudoku-rules/) - [How to Solve Killer Sudoku: Where to Start](https://sudokuflow.com/guides/how-to-solve-killer-sudoku/) - [Jigsaw Sudoku Rules and How to Start](https://sudokuflow.com/guides/jigsaw-sudoku-rules/) - [6×6 Sudoku: Rules and Tips for Beginners](https://sudokuflow.com/guides/6x6-sudoku-rules/) - [How to Solve Samurai Sudoku: Where to Start](https://sudokuflow.com/guides/how-to-solve-samurai-sudoku/) - [Is Sudoku Math? Logic, Not Arithmetic](https://sudokuflow.com/guides/is-sudoku-math/) - [Why Medium Sudoku Suddenly Feels Harder](https://sudokuflow.com/guides/why-is-medium-sudoku-harder/) - [Can a Sudoku Have Two Solutions?](https://sudokuflow.com/guides/can-sudoku-have-two-solutions/) - [How Long Does a Sudoku Take? Times by Level](https://sudokuflow.com/guides/how-long-does-sudoku-take/) - [How to Make a Sudoku Puzzle, by Hand or by Program](https://sudokuflow.com/guides/how-to-make-a-sudoku/) - [16×16 Sudoku: Rules and How to Start](https://sudokuflow.com/guides/16x16-sudoku-rules/) - [Sudoku for Seniors: Large Print Puzzles and How to Start](https://sudokuflow.com/guides/large-print-sudoku-for-seniors/) - [Wordoku: Sudoku with Letters Instead of Numbers](https://sudokuflow.com/guides/wordoku-letter-sudoku/) - [What each Sudoku difficulty level asks of you](https://sudokuflow.com/data/sudoku-difficulty-levels/) - [Sudoku facts](https://sudokuflow.com/guides/sudoku-facts/) - [Sudoku checker](https://sudokuflow.com/tools/sudoku-checker/) - [Sudoku for kids](https://sudokuflow.com/printables/kids/) - [Find the Mistake Sudoku](https://sudokuflow.com/find-the-mistake/) ![Sudoku app icon](https://sudokuflow.com/assets/app-icon.png) ## Want more boards like this? We also make a Sudoku app for iPhone, iPad, Mac and Android. There are no ads in it at all, it works offline, and when you ask for a hint it shows you the technique instead of just filling in a number. [App Store iPhone, iPad, Mac](https://apps.apple.com/app/apple-store/id6502922368?pt=125379530&ct=sudokuflow_web&mt=8) [Google Play Android](https://play.google.com/store/apps/details?id=com.rubigogames.sudoku&referrer=utm_source%3Dsudokuflow%26utm_medium%3Dweb%26utm_campaign%3Dsite) Sudoku Flow is made by Rubigo Games, the developer of the Sudoku app recommended on this site.