---
title: "Sudoku pencil marks: track and update candidates"
description: "A small note records a possibility. Learn how candidates differ from placements and how to keep them up to date."
lang: "en"
canonical: "https://unisu-sudoku.app/en/sudoku-techniques/pencil-marks/"
---

[Back to the learning guides](https://unisu-sudoku.app/en/learn/)

Fundamentals

# Sudoku pencil marks: track and update candidates

A small note records a possibility. Learn how candidates differ from placements and how to keep them up to date.

![Unisu lesson on candidates and notes, showing the explained grid](https://unisu-sudoku.app/_astro/en.arjGw402_Zv79bM.webp)

Candidates are the digits still allowed in an empty cell by the filled numbers in its row, column and box. Writing a candidate does not mean the digit will end up there.

## A small example

If those three areas leave only 3 and 5, note both. Later, a certain 3 is placed elsewhere in the same row. Remove 3 from this cell’s notes. If only 5 remains, you have a Naked Single.

## Recheck after a move

A placement affects its row, column and box. Update the notes there. A missing possibility can create a false pattern; an outdated note can hide a valid move.

## A display is not a proof

A teaching image may show only the digit being discussed. Other hidden candidates have not automatically been eliminated. A technique depends on the actual candidate state. The candidates-and-notes lesson in Unisu gives you practice in adding and updating notes.

## Frequently asked questions

### Are small notes already answers?

No. They show possibilities for an empty cell. A justified placement is what fixes its value.

### Which digits can I write as candidates?

Digits still allowed by the entries in that cell’s row, column and box. Remove candidates already ruled out logically.

### Should I erase every note after a placement?

No. Update the affected row, column and box. Notes elsewhere remain unless their justification has changed.

### Can a missing note create a false Single?

Yes. Forgetting an allowed candidate can make a cell appear unique. Check the complete possibilities before entering its last visible note.

### Why do lesson images sometimes show only selected candidates?

They highlight the digits relevant to that step. A note hidden by the illustration has not necessarily been eliminated logically.

## Keep practicing in Unisu

Download Unisu for iPhone or Android. Solve a puzzle or work through a guided lesson on the technique.

-   [Download on the App Store](https://apps.apple.com/us/app/id6775824061?l=en)
-   [Get it on Google Play](https://play.google.com/store/apps/details?id=app.unisu.sudoku&hl=en)

## Useful before this guide

### How to play Sudoku: rules and your first move

[Read more: How to play Sudoku: rules and your first move](https://unisu-sudoku.app/en/sudoku-rules/)

## Where to go next

Fundamentals

Unisu · Oct 27, 2026

### [Naked Single: One candidate left in a cell](https://unisu-sudoku.app/en/sudoku-techniques/naked-single/)

A Naked Single is a cell with exactly one candidate remaining. Its row, column and box rule out every other digit.

Unisu

### [Learn Sudoku: a path from rules to techniques – Unisu](https://unisu-sudoku.app/en/learn/)

Start with the rules, get comfortable with pencil marks and choose a technique for the situation you’re working on.

Download app

## Ready for a puzzle? Take a moment.

Download Unisu on your phone. Start a Sudoku or choose a lesson on the technique you want to understand.

-   [Download on the App Store](https://apps.apple.com/us/app/id6775824061?l=en)
-   [Get it on Google Play](https://play.google.com/store/apps/details?id=app.unisu.sudoku&hl=en)

![The daily challenge in Unisu](https://unisu-sudoku.app/_astro/en.C-sYR9hJ_1QwI9G.webp)

![A classic Sudoku game in Unisu](https://unisu-sudoku.app/_astro/en.C6DJ1kiE_2dRWDN.webp)