Lesson 3 · Pencil marks
Candidate Notes
Use small candidate numbers to record what can still fit in an empty cell.
See the logic move
Use Next to reveal each reason. On the final step, tap the highlighted cell to complete the move yourself.
Step 1 of 3
Focus
row 3, column 7 currently allows 1, 3, 4, 5, 7.
Select an empty cell to see its legal candidates and why the other digits are blocked.
- 1
row 3, column 7 currently allows 1, 3, 4, 5, 7.
- 2
Inspect R3C7 and then check its row, column, and 3x3 box.
What the board is showing
Explain it simply
Candidates are tiny reminders, not guesses. Remove each one only when logic proves it cannot fit.
Step by step
- List numbers missing from the row.
- Remove numbers seen in the column.
- Remove numbers seen in the box.
- Keep only the possible candidates.
From demonstration to independent solving
Prove the move yourself
What you should be able to do
Create a candidate list by elimination and understand that a remaining candidate is possible, not automatically correct.
Read the exact board evidence
For R3C7, first list the digits missing from row 3. Then remove any of those already in column 7 and the top-right box. The survivors—1, 3, 4, 5 and 7 on this board—are the complete legal list, so 2 is never written as a guess.
When the logic is strong enough
A note belongs in a cell only if the digit is absent from the row, column and box. Remove a note only after a placement or named technique supplies new evidence.
Train the pattern without guessing
Pick one empty cell and calculate its notes twice: once row-first and once box-first. Both routes must produce the same final candidate set.
Quick check: Should a candidate be removed because it feels unlikely?
No. Remove it only when a Sudoku rule or proven technique blocks it.
Turn logic into skill