What are pencil marks in Sudoku?

Pencil marks (also called candidates or notes) are small numbers written in empty cells to indicate which values are still possible in that cell. They help you keep track of possibilities so you do not have to hold everything in your head.

When should I use pencil marks?

Use pencil marks whenever a puzzle is too complex to solve by scanning alone — typically for Medium difficulty and above. For Easy puzzles you can often solve without them, but they are essential for Hard and Expert puzzles where advanced techniques require seeing all candidates.

Should I fill in pencil marks for every cell?

For beginners, it is helpful to fill in candidates for all empty cells (a "full notation" approach). As you gain experience, you may switch to partial notation — only marking candidates in areas where you are actively working. Both approaches are valid.

How do pencil marks help with advanced techniques?

Advanced techniques like Naked Pairs, X-Wings, and Swordfish require you to see all possible candidates in a row, column, or box. Without pencil marks, these patterns are virtually impossible to spot. Pencil marks transform a puzzle from a memory challenge into a visual pattern-matching exercise.

How do I use pencil marks on Sudoku247?

Toggle notes mode by clicking the pencil icon or pressing N on your keyboard. While notes mode is active, clicking a number adds or removes it as a small candidate in the selected cell. Toggle notes mode off to enter final answers.

Sudoku Pencil Marks Guide — How to Use Notes & Candidates

What Are Pencil Marks?

Pencil marks — also called candidates, notes, or possibles — are small numbers written inside empty Sudoku cells to indicate which values could still go there. If a cell could contain a 2, 5, or 7, you write those three numbers in small text inside the cell.

While beginners can often solve easy puzzles by scanning rows, columns, and boxes alone, pencil marks become essential as puzzles get harder. They transform Sudoku from a memory game into a visual logic puzzle — and that is when the real fun begins.

Why Pencil Marks Matter

Without pencil marks, you are relying entirely on working memory to track possibilities. For an easy puzzle with 35+ given numbers, that is manageable. But a hard puzzle may have only 22-25 givens, meaning you need to track candidates across 55-60 cells. That is far beyond what any human memory can handle reliably.

More importantly, advanced solving techniques require you to see candidates. Techniques like Naked Pairs, X-Wings, and Swordfish work by identifying patterns in how candidates are distributed across rows, columns, and boxes. Without written candidates, these patterns are invisible.

How to Fill In Pencil Marks: Step by Step

There are two main approaches to pencil marking. Both are valid — choose the one that matches your style and the puzzle's difficulty.

Method 1: Full Notation

Full notation means writing every possible candidate in every empty cell before you start solving. This is the most thorough approach and is recommended for beginners and for Expert-level puzzles.

1
Pick a cell
Start with any empty cell — top-left corner works fine.
2
Scan the row
Which numbers 1-9 are already placed in this row? Cross them off mentally.
3
Scan the column
Cross off any numbers already in the same column.
4
Scan the box
Cross off any numbers already in the same 3x3 box.
5
Write the remaining numbers
These are the candidates — write them as small pencil marks in the cell.
6
Repeat for every empty cell
Full notation takes a few minutes upfront but makes the rest of the solve much faster.

If any cell ends up with only one candidate, you have found a Naked Single — fill it in immediately.

Method 2: Snyder Notation

Named after world-class puzzle creator Thomas Snyder, Snyder notation is a minimalist approach that keeps the grid cleaner. Instead of marking all candidates everywhere, you only mark a candidate in a cell when that number can appear in exactly two cells within a 3x3 box.

For example, if the number 7 can only go in two cells within a box, write a small 7 in both of those cells. If 7 could go in three or more cells in the box, do not mark it yet.

The advantage of Snyder notation is that it immediately reveals Naked Pairs and Pointing Pairs without cluttering the grid. Many competitive solvers use Snyder notation for the first pass, then switch to full notation when they need to apply more advanced techniques.

Pencil Mark Best Practices

Over years of solving, experienced players have developed habits that make pencil marking more effective:

Best Practices

Update immediately. Every time you place a number, scan its row, column, and box to remove that number from all affected cells' candidates. Stale pencil marks lead to errors.
Use consistent positioning. Many solvers write candidates in a fixed 3x3 mini-grid within each cell — 1 in the top-left, 2 in the top-center, and so on. This makes it faster to scan for specific numbers.
Look for cells with few candidates. Cells with only 2-3 candidates are the most useful. They are likely to yield Naked Singles after a single elimination.
Scan by number, not by cell. Try filling candidate 1 everywhere, then candidate 2 everywhere. This is faster because you only need to scan for one number at a time.
Do not be afraid to erase. The whole point of pencil marks is that they are temporary. Clean candidates are correct candidates.

Pencil Marks and Advanced Techniques

Pencil marks unlock the full power of advanced Sudoku techniques. Here is how they connect:

Naked Single: A cell with only one candidate. Impossible to spot without pencil marks in complex puzzles.
Hidden Single: A candidate that appears in only one cell within a row, column, or box. Pencil marks make these visible at a glance.
Naked Pair: Two cells in the same unit sharing the same two candidates. With pencil marks, pairs literally jump out at you.
Pointing Pair: When a candidate within a box is restricted to one row or column, you can eliminate it from the rest of that row or column. Only visible with candidates written out.
X-Wing: A pattern where a candidate appears in exactly two positions in two different rows (or columns), forming a rectangle. Without full notation, X-Wings are nearly impossible to find.
Y-Wing: Three cells forming a chain of pairs that eliminates a candidate from cells seeing both endpoints. Requires precise candidate tracking.
Swordfish: An extension of X-Wing across three rows and columns. Absolutely requires full notation to identify.

Using Pencil Marks on Sudoku247

On Sudoku247, pencil marks are built into every puzzle. To use them:

Select a cell by clicking or tapping it.
Toggle notes mode by clicking the pencil icon in the toolbar or pressing N on your keyboard.
Click a number (or press 1-9) to add or remove it as a candidate in the selected cell.
Toggle notes mode off to return to normal entry for placing final answers.

Our AI-powered solver also uses pencil marks internally — when you view the step-by-step solution, it shows you exactly which candidates were considered and eliminated at each step.

Common Pencil Mark Mistakes to Avoid

Ready to practice? Start with a Medium puzzle and try full notation — you will be amazed at how much easier the puzzle becomes when you can see all the possibilities laid out. For more solving strategies, explore our complete techniques library.