Sudoku pencil marks should answer a question
Small numbers help when scanning the filled digits no longer gives you a clear move. Good Sudoku pencil marks show what is still possible in a cell. They let you compare possibilities across a whole row, column or box without repeatedly rebuilding them in your head.
A pencil mark is not an answer. Writing 4 in a cell’s notes says “4 has not yet been ruled out here.” Entering a full-sized 4 says “this cell is 4.” Use the game’s Notes control to keep these actions distinct. The selection and small-number layout help you see which mode is active before pressing a number.
If you are new to the rules, begin with the first-move guide. This page starts from the point where you know how the three constraints overlap and want your notes to make the next deduction easier.
Build complete Sudoku pencil marks before using absence as evidence
For a cell, take the digits 1–9 and remove anything already in its row, column or box. What remains is the cell’s initial candidate list. Later sound deductions may shorten that list. Until then, a digit that fits all three regions remains possible.
Imagine that a cell’s actual candidates are 3 and 6. If you remember to write only 3, your notebook displays a single digit but the puzzle has not supplied a single possibility. Filling 3 from that incomplete list could happen to be correct. It would still lack the reason needed to make the move safely.
That is the most useful distinction in Sudoku pencil marks: not written and ruled out mean different things. Before declaring a single, inventory all nine digits for the target cell. Before declaring that a digit has one position, check every empty cell in the relevant unit.
Board overview
View the whole grid first. Zoom in to read candidates and their coordinates.
This figure contains a valid hidden single. Its proof survives the visible 6. Erasing the 6 merely to make the cell look solved would reverse cause and effect: establish the row argument first, then fill 3 and clear the cell’s notes.
When to expand your Sudoku pencil marks
On an Easy board, you may need few notes. Start by scanning units with several filled cells. When a cell or region repeatedly makes you rebuild the same options, record them. On a Medium or Hard board, complete notes across more of the grid make comparisons much easier.
Partial notes are useful as a memory aid when you know they are partial. For example, you might record only the possible positions of digit 8 while scanning boxes. Do not then treat the absence of 2, 4 or 7 as proof. Switch to a complete candidate inventory before using pairs, triples or a rectangle pattern.
There is no benefit in covering the board with numbers you never examine. Keep the task small: pick a unit, fill in its complete candidates, look for a specific deduction, and update it after your next entry. Widen the area when the technique you are considering requires it. An X-Wing, for instance, requires complete positions of one digit in two entire rows or columns.
Keep Sudoku pencil marks in fixed positions
The game places candidates 1, 2 and 3 along the top of a cell, 4, 5 and 6 in the middle, and 7, 8 and 9 along the bottom. A candidate keeps its position even when others disappear. That lets you scan for the same digit across many cells without reading a changing string.
For paper, use the same tiny 3×3 arrangement or another consistent convention. Leave room to erase. Distinguish your notes from the fixed clues with size, not just color; the board must remain legible when printed in black and white.
The game’s automatic logical candidates used by Hint are separate from your manual notes. Missing or mistaken player notes do not become facts for the hint engine. This makes hints dependable while leaving you free to use a lighter personal note-taking style.
Clean only the neighbors after a placement
Suppose you place 5 at r1c5. Remove candidate 5 from every other empty cell in row 1, column 5 and its top middle box. Clear the placed cell’s own notes. Do not erase 5 elsewhere simply because you have entered one 5: other rows, columns and boxes still need it.
Board overview
View the whole grid first. Zoom in to read candidates and their coordinates.
With automatic note cleanup enabled, entering a number and removing its neighboring notes are one action. Undo restores both the entry and the notes that action removed. If cleanup is disabled, make those removals yourself. After erasing or correcting an old entry, review nearby notes: possibilities excluded by that entry may need to return.
A pair turns Sudoku pencil marks into a deduction
In the next verified position, r9c7 and r9c9 both have exactly the candidates 1 and 9. They share row 9. One must be 1 and the other 9, in an order you do not yet know. Together they occupy those two digits in the row, so no other cell in row 9 can contain either.
Board overview
View the whole grid first. Zoom in to read candidates and their coordinates.
Purple cells give the evidence; a red cross marks a candidate this step can remove.
Board overview
View the whole grid first. Zoom in to read candidates and their coordinates.
Notice the limited conclusion. You may delete certain candidates outside the pair. You may not choose the order of the pair, delete the digits from unrelated rows, or discard another candidate from one pair cell without a separate reason.
Board overview
View the whole grid first. Zoom in to read candidates and their coordinates.
Dashed borders mark changed givens; inspect the purple cells to see why the original rule no longer applies.
The same-looking two written notes do not establish a pair if either cell also has an unrecorded possibility. Accurate Sudoku pencil marks preserve the alternatives until a rule removes them. That discipline prevents both false singles and false pairs.
Exercise 1: which notes must disappear?
Study the pair shown in row 4 of the exercise. Its two cells each have candidates 2 and 4. Select one candidate outside the pair that the row argument lets you remove. The exercise accepts any one of the valid deletions, then shows the whole step.
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Answer: remove 2 and 4 from r4c1, remove 4 from r4c2, and remove 2 and 4 from r4c5. The pair at r4c4 and r4c7 occupies both digits in row 4. Neither pair cell is individually solved by this step.
Exercise 2: is one written mark enough?
Inspect the second position and identify the certain placement in column 4. In your explanation, decide whether the cell’s own list or the digit’s unit-wide position supplies the evidence.
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Answer: place 2 at r1c4. Its candidates are 2 and 5, so a notebook showing just 2 would be incomplete. The valid reason is that no other cell in column 4 can contain 2. Compare the two types in Naked Single vs. Hidden Single.
Return to the board with a useful habit
Before applying a candidate deduction, ask what would have to be complete for its proof to work. A naked pair needs both cells’ full lists. A hidden pair needs all positions of both digits in the unit. A pattern that crosses rows needs the relevant rows’ full positions. Verify that area, then make the smallest justified change.
Use Sudoku pencil marks to expose evidence, and use a staged hint when you need help finding that evidence. The game saves your entries and notes on this device. Open a Medium board, make one elimination you can explain and return to singles afterward. Candidate work is valuable because it makes a later placement certain.