How to Use Notes and Pencil Marks in Sudoku

What notes actually are, and when to use them

Notes, also called pencil marks or candidates, are the small shapes you jot inside an empty cell to record which values could still legally go there. They are not answers, and they are never counted as final moves or mistakes. They are a memory aid, and that is the whole point of them. Instead of recalculating the possibilities for a cell every time your eye passes over it, you write them down once and let the board hold the information for you, which frees your attention to look for patterns across the whole grid. On easy boards you often do not need notes at all, because a quick scan reveals a value that can only sit in one place. The moment a puzzle makes you pause and genuinely think, that is the signal to turn Notes mode on. A good trigger is any cell where you can narrow the options to two or three shapes but cannot yet prove which one is correct, so rather than guessing, you record the candidates and move on.

How to add and remove a note

1. Turn on Notes mode with the Notes toggle, so taps become pencil marks instead of final answers.
2. Select an empty editable cell that you cannot yet solve outright.
3. Tap a shape to add it as a small candidate, and tap the same shape again to remove it if you rule it out.
4. Turn Notes mode off when you are ready to place a final shape as your committed answer.

A quick worked example

Take an empty cell at r5c5 sitting in the middle box of the board. Glancing along row 5 you rule out four shapes that already appear there, and scanning down column 5 removes two more from consideration. That leaves three possible shapes, so you mark all three as small notes inside r5c5 rather than trying to hold them in your head while you look elsewhere. You do the same for its neighbor r5c6 and discover it shares only two of those same candidates. Now the board is showing you its structure instead of hiding it, and the next few placements will do the deciding for you. A later shape dropped into the box may knock out one of the three notes in r5c5, leaving two, and a placement further along the row may knock out another, leaving a single confident answer that you can commit to without any doubt at all. That is the quiet power of pencil marks in practice: they do not solve the puzzle for you, but they lay the whole logic out plainly where your eye can follow it, so that the correct move becomes obvious the very moment it actually exists rather than staying hidden in a cell you keep passing over.

Let smart cleanup do the chores

The genuinely tedious part of notes is keeping them current as the board changes. Every time you place a final shape, that value instantly becomes impossible everywhere else in its row, its column, and its box, which means the matching candidate should be erased from all of those cells right away. Doing this by hand is slow and very easy to forget, and a single stale note can quietly send you down a wrong path that takes ages to unwind. Shapedoku can handle the erasing for you: when you place a shape, it removes that same candidate from the notes of every cell it affects, so your pencil marks always reflect the true state of the puzzle. You can switch the feature off in settings if you prefer to manage everything by hand, and you can always add a cleared note back if you decide you want it. Either way, the goal is the same, notes you can actually trust.

Turning notes into answers, and avoiding traps

Once your notes are accurate, solving becomes a matter of watching them shrink. When a cell is reduced to a single candidate, that candidate is the answer and you can place it with full confidence. There is a second pattern worth learning too: when a shape appears as a candidate in only one cell of a given box, row, or column, it must belong there even if that cell still lists other options alongside it. Clean, trustworthy notes turn both of these moments into instant, satisfying placements. The mistakes to watch for are the mirror image of good habits, so avoid forgetting to remove a candidate after a placement, avoid filling every cell with notes at once and cluttering the board, and avoid trusting an old note you never updated. Add marks only where you actually need them, keep them current, and let smart cleanup carry the load whenever you would rather focus on the solving itself. Start a Medium or Hard board and try it in action.