Sudoku Notes: When and How to Use Pencil Marks

Sudoku notes track the digits a square can still hold. When you need them, two ways to write them, and a real puzzle where notes find the move.

Updated 2026-11-06

When to start taking notes

When no square has a single digit left and no digit has a single place left in a row, column or box, it is time to write notes. In our app that never happens on Easy or Medium. On Hard it always happens at some point: every Hard puzzle needs at least one step that removes candidates rather than placing a digit.

Two ways to write notes

Full notes: in every empty square, write each digit that could still go there, small, always in the same spot (1 top left, 9 bottom right). It takes a while, but nothing is missed.

Corner marks: write a digit small in the corners of a square only when that digit has two places left in a box. It is quicker, and the marks point straight at useful pairs. Our app has both: its Classic note style puts each digit in its own spot, and the Corner + Center style adds corner marks, with centre notes for what a square can hold.

Notes at work on a real puzzle

This is puzzle 1 on Hard sheet 1 of our printable Sudoku, after every single has been placed: the digits in blue were filled in, and now no square has one digit left and no digit has one place left. With full notes written in, a pattern shows. In one box, the 8 can only go in two squares, row 1, column 7 and row 2, column 7, marked on the board, and both are in column 7. So the 8 of that line is in this box, and it cannot go anywhere else in column 7. The note 8 comes out of row 7, column 7, which leaves that square with one digit: 7.

364289125858959712568968738936948973464561256453452146714635983596185934724573457924848248635124734685978168459451734682456956894579861421435793459359

Hard, sheet 1, puzzle 1, with full notes: the struck-out note is the one the marked squares rule out.

Pointing Pair →

Keeping notes tidy

Each time you place a digit, take it out of the notes in its row, its column and its box.
Check your notes after a break: one note that should have gone can hide the next move.
On paper, write notes small and light, so they are easy to rub out.
If the notes get messy, rewrite them for one box at a time.

Stuck on a Hard or Expert Sudoku? The Technique You Need Next →

Practice

Every Find the Mistake puzzle on this site has exactly one wrong digit, and you can play each board right on its page.

Play the puzzles →