When to start using pencil marks

By David, 26 September 2026

Pencil marks, which this site calls notes, are the small numbers you write in a square to remind yourself what could go there. Every serious solver uses them. But writing notes too early, or too many, can slow you down on an easy puzzle and hide the answer on a hard one.

Here is a simple way to think about it.

Easy and Medium: mostly no notes

On the easier levels, the board has enough numbers that scanning does the work. Pick a number, look at a box that is missing it, and see which squares are ruled out by the rows and columns around it. Usually only one square is left.

If you fill every square with notes on an Easy puzzle, you spend more time writing than solving. Save notes for the rare square where you want to remember two options.

Hard: notes for the tricky areas

On Hard puzzles, scanning takes you a long way and then stops. That is the moment to start writing notes, but only in the part of the board you are working on. A box or a row with three or four empty squares is a good place to begin.

Look for two things once the notes are in:

• A square with a single note. That is your answer.
• Two squares in a unit with the same two notes. That is a naked pair, and it lets you clear those two numbers from every other square in the unit.

Expert and above: notes everywhere

From Expert upwards, you will almost always need notes on every empty square. The patterns that crack these puzzles, such as pointing pairs, triples and the X-Wing, can only be seen when the candidates are written down.

A good way to fill them in is number by number rather than square by square. Take the 1s: for every box without a 1, mark each square where a 1 could go. Then the 2s, and so on. It is quicker, and you are less likely to miss one.

Keep them honest

Notes are only useful if they are right. Out-of-date notes are the most common reason people get stuck on a puzzle they could solve.

This site helps in one way: when you place a correct number, that number is removed from the notes in the same row, column and box automatically. But it does not clean up after techniques like pairs. That part is yours, and it is where the puzzle is actually solved.

If you are stuck, go back over the notes around the last few numbers you placed. More often than not, a candidate that should have gone is still there.

A quick tip on the controls

Press N on a keyboard, or tap Notes, to switch between answers and notes. The button shows ON or OFF so you always know which mode you are in. Tap a note again to remove it. Undo takes back notes as well as answers.

Ready to try it? Start a Hard puzzle and hold off on notes until scanning runs dry.

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