Pencil Marks
Sudoku with pencil marks and candidate notes
Pencil marks let you hold possible numbers inside each empty cell. Ruedoku supports visible candidate notes so you can solve harder puzzles without keeping the entire search space in your head.
How to use pencil marks well
Start by marking candidates only for cells that are genuinely ambiguous. If a row, column, or box already rules out most digits, the remaining marks tell you where to focus.
After every placement, update related cells. Ruedoku helps by keeping candidate mode close at hand, and hints can explain which marked cell should resolve next.
Reduce clutter
Remove impossible candidates as soon as a digit is placed in a peer row, column, or box.
Find singles
A cell with one remaining candidate is a naked single. A unit with one place for a digit is a hidden single.
Spot patterns
Pencil marks make locked candidates, pairs, and X-Wing rectangles easier to verify.
Keyboard workflow
- Select an empty cell with the mouse, tap, or arrow keys.
- Press c to show candidate mode.
- Use Shift plus 1-9 to toggle pencil marks in the focused cell.
- Press 1-9 without Shift when you are ready to place a final value.
FAQ
What are pencil marks in Sudoku?
Pencil marks are small candidate notes inside empty cells. They show which numbers might still fit before you choose a final placement.
How do I toggle pencil marks in Ruedoku?
Turn on Candidates from the toolbar, or press c on a keyboard. With keyboard entry, Shift plus a number toggles that candidate note in the selected cell.
When should I use pencil marks?
Use pencil marks for medium and hard puzzles, especially when simple scanning no longer reveals the next number.