How to Use Pencil Marks

Pencil marks are the small candidate numbers you jot into an empty cell to record which digits could still go there. They aren't a solving technique in themselves — they're the foundation every technique beyond simple scanning is built on. Learn to keep them well and the whole middle-to-hard game opens up.

What are pencil marks?

When you can't yet place a digit for certain, you note all the digits that are still legal for that cell — its candidates. On SudokuStreak you switch to pencil mode with the Notes button (or press P) and tap the numbers in. A cell showing a small 3, 5 and 8 is telling you: "one of these three, not decided yet."

Why they matter

Almost every technique past the beginner stage reads pencil marks rather than the grid. A naked pair is two cells whose pencil marks match. A hidden pair, an X-Wing, a Swordfish — none of them are visible until the candidates are on the board. Without pencil marks you're limited to what you can hold in your head, which runs out fast on Hard and above.

Keeping them tidy

1. Only mark when you're stuck. On Easy and much of Medium you can place cells directly — don't bother pencilling those.
2. Update after every placement. Each digit you write removes itself as a candidate from its row, column and box. Keep marks honest and they never lie to you.
3. Try Snyder notation. If full marks feel like a wall of numbers, Snyder notation keeps only the useful ones and stays far cleaner.

Common mistakes

• Letting them go stale. An out-of-date candidate list is worse than none — it hides real deductions behind numbers that should be gone.
• Marking everything from the start. Fill the easy cells first; only pencil the regions that resist.
• Trusting a mark you didn't check. If a technique's conclusion surprises you, re-verify the candidates in those cells before acting.