How to use notes mode (pencil marks) in sudoku

By Miguel Arechaga

When a sudoku moves up in level, there comes a point where just looking at the board isn't enough anymore: there are too many possibilities to keep in your head. That's where notes mode comes in, also called pencil mode or candidate marks. It's the tool every expert player uses, and on MundoSudoku it's built right into the game. In this guide I'll explain what notes are, how to turn them on on the site, and, above all, how to use them to solve hard sudokus without guessing.

What are notes or pencil marks?

Notes are small numbers you write inside a cell to remember which values could go there. Instead of placing a final number, you jot down all its candidates in miniature, usually in a mini-grid inside the cell itself. That way you don't have to memorize anything: the board remembers for you. When you later discover a candidate is impossible, you cross it out; and when a cell has only one note left, that's the answer.

Think of it as writing in pencil before writing in pen. Notes are provisional and don't count as an answer: they're your scratch pad inside the board.

How to turn on notes mode on MundoSudoku

We've made it as simple as possible. Right above the number pad you'll see a button with a pencil that says ✏️ Notes. It works like this:

• It starts off (shows OFF). In that state, pressing a number writes the final answer, as usual.
• Press it once to turn it on: it turns yellow and shows ON. Now, every number you press gets added as a small note in the selected cell.
• Press it again to turn it off and go back to placing final numbers.
• If you prefer the keyboard, you can toggle notes with the N key.

The button looks and works the same on mobile, tablet, and desktop, so you can note candidates comfortably wherever you're tapping.

How to use it, step by step

1. Turn on notes mode by pressing the pencil button (or the N key).
2. Select the empty cell you want to mark.
3. Tap the candidate numbers, one by one. Each tap adds or removes that note, so if you make a mistake, tap the same number again to erase it.
4. Once you're sure of a cell's answer, turn off notes and write the final number. Doing so automatically clears the notes in that cell.

On top of that, to save you work, when you place a final number the game automatically removes that candidate from the notes in its same row, column, and block. It's the same behavior you'll find in the best sudoku apps: every answer cleans up its own notes that no longer make sense.

The candidate technique: where to start

Noting candidates doesn't mean filling every cell with every number; that only clutters the board. The smart way to use notes is this: when basic scanning stalls, pick a row, column, or block that already has plenty of numbers, and note, only in its empty cells, the few values that still fit. Often you'll discover that a number can only go in one cell of that group, even if that cell had several notes. That's a hidden single, and it's impossible to spot at a glance without notes.

Pairs and triples: what notes let you see

The real magic of notes shows up with patterns. If in the same block there are two cells whose notes are exactly the same two numbers — say, both can only be 4 or 7 — then that 4 and that 7 are "reserved" for those two cells. You can erase the 4 and the 7 from the notes in the rest of the block. This is called a naked pair, and without written notes it's nearly impossible to spot. The same applies to triples. If you want to dig deeper into these patterns, we cover them in our advanced sudoku techniques guide.

Mistakes worth avoiding with notes

• Noting everything at once. Filling all 81 cells with candidates is overwhelming. Only note where scanning stops making progress.
• Not updating them. A note is only useful if it's current. Every number you place should cross out candidates; that's why the site's automatic cleanup helps so much.
• Confusing a note with an answer. A cell with three notes isn't solved. Don't treat it as done until only one candidate is left.

Is using notes cheating?

Not at all. Notes have always been a legitimate part of sudoku: on paper they're written in pencil in the corners of the cell. They don't give you the answer, they just help you organize the logic you'd have to do in your head anyway. In fact, learning to use them well is the step that separates someone who can only solve easy sudokus from someone who can handle hard and expert levels. And since it's optional and comes turned off by default, if you prefer the challenge of playing purely from memory, just leave the pencil off.