“I never use pencil marks, otherwise it's cheating.” Spend any time in Sudoku forums or groups and you will have come across this line. It comes from a good place: the idea that a real solver ought to keep everything in their head.
But it rests on a misunderstanding. And above all, it traps whoever believes it under a glass ceiling: hard puzzles will stay permanently out of reach. Let's set the record straight.
Where the myth comes from, what the rules of Sudoku really say, the list of techniques that become plainly impossible without pencil marks, and the essential distinction between noting and guessing.
Part 1
Where does the myth come from?
It has three sources, and none of them is absurd — which is exactly why it persists.
The newspaper Sudoku
Most people discover Sudoku through an easy grid at the back of a newspaper, pen in hand. At that level you really do fill everything in from memory, and that is perfectly fine. The habit sets in, then slowly hardens into a moral norm: you are supposed to do it in your head.
Confusion with trial and error
Placing a digit at random to see whether the grid holds, then erasing when it breaks: that is trial and error. Many solvers consider it an admission of defeat, and that is a legitimate debate. The trouble is that in many minds “writing several digits” and “trying at random” have become the same thing. They are in fact opposites, as we'll see in part 4.
The feeling of failure
“If I need to write things down, it means I can't do it.” Yet nobody criticises a chess player for calculating a line before moving, or a mathematician for writing out the steps of a calculation. Writing is not an admission of weakness: it is a working tool.
Part 2
What the rules actually say
The rules of Sudoku fit in a single sentence:
Each digit from 1 to 9 appears once per row, once per column and once per 3×3 box.
That is all. There is no second rule. Nothing about what you are allowed to write in the cells, nothing about pencils, nothing about memory. The only thing that counts is that the final grid is valid and was reached by deduction.
So “you must not use pencil marks” is not a rule of the game. It is a constraint some solvers impose on themselves: a perfectly respectable personal choice, but one that binds nobody else.
Marking candidates is entirely standard practice there. On timed puzzles where every second counts, if pencil marks cost time nobody would use them. It is precisely the opposite: they are what makes speed possible on hard grids.
Part 3
The techniques that are impossible without them
This is where the argument becomes unanswerable. Take any technique beyond beginner level: they all work on candidates, never on the digits already placed.
- Naked pairs and hidden pairs — A naked pair is a cell left with only 2 candidates. Without notation the notion has literally no meaning.
- X-Wing — You need to spot a digit that can only go in 2 cells across two different rows, aligned in columns. Without the candidate map, that configuration is simply invisible.
- Y-Wing — The whole technique rests on bivalue cells, that is, cells with exactly 2 candidates. The word itself means nothing if you do not write them down.
- W-Wing — More demanding still: you need two identical bivalue cells, plus a strong link located elsewhere in the grid.
- Swordfish, XY-Chain, 3D-Medusa — These techniques consist literally of following chains of candidates across the whole grid.
One cell, two ways of seeing it
The same cell, depending on whether you write the candidates or not:
An empty cell among others. No usable information: impossible to tell whether it takes part in any configuration.
A bivalue cell {2,7}: the mandatory starting point of a Y-Wing or a W-Wing.
On the right, the information was already there in the grid. The pencil marks merely make it readable.
The conclusion is inescapable: refusing to use pencil marks from level 6-7 onwards is not purism. It is deliberately giving up almost the entire toolbox, then concluding that the puzzle is “too hard”. It is not: it is the toolbox that is empty.
Part 4
Noting is not guessing
Here is the distinction that settles the debate for good.
- Guessing means placing a digit without proof, hoping the grid holds.
- Noting means making visible information that already follows from the rules.
When you write {2,7} in a cell you are inventing nothing at all. You are simply observing that the other seven digits are already eliminated by the row, the column and the box. That information existed before you wrote it down — you are merely making sure you do not lose it.
It is the difference between writing out your working and calling out a number at random. Trial and error injects an unproven hypothesis into the grid; pencil marks add strictly nothing that was not already deducible. They relieve your memory, not your reasoning.
Marking every candidate in every cell from the first minute is counter-productive: you spend ten minutes filling in an unreadable grid that you then have to correct constantly. The right method is progressive: start with the basic techniques and only mark candidates once simple scanning stops producing results.
Part 5
So should you always use them?
No, and it would be dishonest to claim otherwise. It depends entirely on the level of the puzzle.
- Levels 1 to 4 — Pencil marks are unnecessary. Doing without them is in fact excellent training for visual scanning, a skill that pays off later.
- Levels 5 and 6 — They become a genuine time-saver, especially on end-of-grid deadlocks.
- Levels 7 and above — They become indispensable, since the required techniques operate directly on candidates.
In other words: pencil marks are not a crutch for weak solvers, they are the working tool of solvers who are improving.
🎬 See pencil marks in action
On the Domino33 channel, over 120 free videos show candidate notation in real situations, technique by technique, from beginner to expert level.
Part 6
Quiz: is it cheating?
Four situations. Decide whether the solver is cheating or not.
❓ Frequently asked questions
No. The rules fit in one sentence: each digit from 1 to 9 appears once per row, once per column and once per box. There is no rule governing what a solver may write while solving.
Guessing means placing a digit without proof, hoping the grid holds. Noting means making visible information that already follows from the rules. Pencil marks create no information: they simply stop you losing it.
Almost all of them beyond beginner level: naked pairs, hidden pairs, X-Wing, Y-Wing, W-Wing, Swordfish, XY-Chain and 3D-Medusa all operate on candidates. The Y-Wing and the W-Wing rely on bivalue cells, a notion that means nothing without notation.
Up to around level 4, doing without them usefully trains visual scanning. From levels 5 and 6 they become a real time-saver. From level 7 they become indispensable.
📋 Summary
Pencil marks in five points
- No rule forbids them — the rules of Sudoku only concern the final grid
- Noting ≠ guessing — notation reveals a deduction, trial and error injects a hypothesis
- Advanced techniques require them — X-Wing, Y-Wing, W-Wing and Swordfish all operate on candidates
- Work progressively — do not mark the whole grid at once
- Levels 1-4 without, 5-6 useful, 7+ indispensable
Next time someone tells you that pencil marks are cheating, you will know what to answer: it is not a rule of the game, it is a belief. And it costs whoever holds it dearly, because it shuts the door on the most interesting puzzles.
📘 Learn to mark candidates, then to exploit them
All 40 techniques in the memento rest on well-managed candidate notation, explained sheet by sheet, from beginner to expert level.
