Sudoku pencil marks: When they help and when they confuse
Organise sudoku notes, distinguish complete candidate lists from reminders and avoid deductions based on missing pencil marks.
Give your notes a clear meaning
Small digits can reduce what you need to remember, but only if you know what they represent. Some players write every remaining possibility in each cell. Others mark only the possible locations for one digit within a box. Both conventions can help organise a puzzle, but missing marks do not mean the same thing in each system. If your list is partial, the absence of an 8 does not prove that 8 is excluded. Before using a candidate technique, establish what information your notes actually contain.
Update after a proven move
When you place a digit, it stops being a candidate in the other cells of its row, column and box. Check those three units in a consistent order. With manual notes, you need not rebuild the whole grid on every turn, but the lists you are about to use must remain reliable. Some eliminations come from a technique, such as a naked pair, rather than a newly placed digit. Remembering the reason for such an elimination makes it easier to review if you undo a move or encounter a contradiction later.
Fewer marks still need discipline
A grid crowded with candidates can be difficult to read. One approach is to begin with visible placements and add complete lists in an area when you need them. If you choose that approach, do not confuse an area with few notes with an area that has few possibilities. Another option is to use complete notes and focus on a single digit or unit at a time. Try both during practice and see which makes your decisions easier to explain. Clarity matters more than copying the appearance of another player’s grid.
Make one small check
Before entering an apparent single, rebuild its candidate list from the three units. Before using a pair, confirm that each of the two cells has exactly those two possibilities. If you use automatic pencil marks, find out whether the system calculates direct exclusions only or also preserves eliminations made through techniques. These are different behaviours and can produce different lists. When uncertain, return to the rule and the explanation for the move. Notes are a tool for representing your reasoning; their value depends on that representation staying consistent with the puzzle.
Now try it on the grid.
Return to an easy puzzle and look for a chance to use what you’ve learned.
Practise with a Sudoku