How to Use Notes and Pencil Marks in Sudoku

What notes are

Notes, also called pencil marks or candidates, are the small shapes you jot inside an empty cell to record which values could still go there. They are not answers. They are a memory aid that lets you see the whole puzzle at once instead of recalculating every option in your head.

When to add notes

On easy boards you often do not need notes at all. As soon as a puzzle makes you pause and think, switch on Notes mode and start marking candidates. A good moment to add them is when a cell has two or three possibilities and you cannot yet prove which one is correct.

How to add and remove a note

1. Turn on Notes mode with the Notes toggle.
2. Select an empty editable cell.
3. Tap a shape to add it as a small candidate. Tap the same shape again to remove it.
4. Turn Notes mode off to place a final shape as your answer.

Smart notes cleanup

The tedious part of notes is keeping them up to date. Every time you place a final shape, that value becomes impossible everywhere else in its row, column and box. Shapedoku can do this cleanup for you. When you place a shape, it removes that same candidate from the notes of the cells it affects, so your pencil marks always reflect reality. You can turn the feature off in settings if you prefer to manage notes by hand, and you can always add a cleared note back.

Turning notes into answers

Watch your notes shrink. When a cell is left with a single candidate, that candidate is the answer. When a shape appears as a candidate in only one cell of a box, row or column, it belongs there. Clean, accurate notes turn these moments into easy, confident placements.

Common mistakes to avoid

• Forgetting to remove a candidate after a placement, which sends you down a wrong path.
• Adding notes to every cell at once, which clutters the board. Add them where you actually need them.
• Trusting an old note you never updated. Keep them current, or let smart cleanup do it for you.

Ready to try notes in action? Start a Medium or Hard puzzle in Shapedoku, switch on Notes mode, and let smart cleanup keep the board clear while you solve.