Pencil marks are the difference between solving and stalling on harder Sudoku boards — but over-marking is its own trap. Here is the discipline.
Pencil marks turn short-term memory into something you can see, which is exactly what a hard Sudoku board demands. Used well they unlock the board; used lazily they bury it.
The difference is not whether you mark, it is when and how much.
Mark only cells with two candidates
The high-value marks are the cells that are down to two possibilities. Those cells form the chains and pairs that crack hard boards, and there are rarely more than a dozen of them.
A cell with five candidates tells you almost nothing. Leave it blank and come back after the constrained cells have been resolved.
Clear the marks you have already spent
The most common failure is stale marks: you place a six but forget to remove six from eleven other cells. Now your board is lying to you and every downstream deduction is suspect.
Make it a habit that placing a digit triggers a sweep of its row, column and box for that digit in pencil form.
Use marks to find pairs, not to store the board
Two cells in the same unit sharing the same two candidates are a naked pair: those two digits belong to those two cells and can be eliminated everywhere else in the unit. That is the payoff.
If your marks are not producing eliminations, you have too many of them. Erase and re-mark the constrained cells only.
FAQ
Does the PlayJoy board support pencil marks?
Yes — the notes toggle lets you mark candidates on touch and desktop, and marks are cleared automatically when you place a conflicting digit in the unit.