Guide · Beginner
Pencil Marks: How to Take Notes in Sudoku
Every solver hits a wall where no number jumps out. Pencil marks are how you climb over it. They turn a puzzle you have to hold in your head into one you can see on the page.
What pencil marks are
A pencil mark, or candidate, is a small number written in the corner of an empty cell. It means “this number could still go here.” A cell with marks 2, 6 and 9 can only be one of those three.
How to write them
- Pick an empty cell.
- Look at its row, its column and its box.
- Write every number from 1 to 9 that does not appear in any of those three places.
Keep them up to date
Pencil marks are only useful when they are correct. Every time you place a number, erase it from the marks of every cell in the same row, column and box. Most mistakes in Sudoku come from forgetting this one step.
What to look for
- A cell with one mark. That is the answer for the cell. This is called a naked single.
- A number with one spot. If a number appears as a mark in only one cell of a row, column or box, it must go there. This is a hidden single.
- Two cells, two marks. If two cells in a row both have only the marks 3 and 8, those two numbers belong to those two cells. You can erase 3 and 8 from the rest of the row. This is a naked pair.
Full marks or smart marks?
Some solvers fill every cell with marks at once. Others only mark cells with two or three options. Full marks are slower to write but make advanced patterns easier to see. For hard puzzles, full marks usually win.
In our game
On Wink Puzzle you can switch to notes mode and tap numbers to add or remove pencil marks, so you never have to erase by hand.
Pencil marks are where Sudoku changes from looking to thinking. Once you are comfortable with them, every technique on our strategies page opens up.