Sudoku Notes - How to Write and Manage Candidate Digits
Pencil marks (candidate notes) are an essential tool for solving intermediate and advanced Sudoku. This article covers efficient note-taking methods, when to update them, and the patterns you can read from well-maintained notes.
When to Start Writing Notes
The right time to start writing pencil marks is when Naked Singles and Hidden Singles have been exhausted. Easy-level puzzles can often be solved without notes, but Medium and above require them. You can either write notes for all cells at once or only where needed. The former is comprehensive but time-consuming; the latter is efficient but risks oversights. The recommended hybrid approach is to write notes only for cells where candidates are narrowed to 2-3.
Rules for Updating Notes
The value of notes depends on their accuracy. Every time a digit is confirmed, you must remove that digit from the notes of all cells in the same row, column, and block. Failing to update risks making incorrect judgments based on outdated candidates. Digital apps offer auto-remove features, but paper solving requires manual updates. The key to preventing missed updates is to immediately scan the notes in the same row, column, and block right after confirming a digit.
Patterns Readable from Notes
Accurate notes enable the discovery of advanced techniques. Naked Pairs (two cells with the same 2 candidates), Pointing Pairs (candidates aligned in a row within a block), and X-Wings (candidates in the same 2 columns across 2 rows) can all be visually recognized as patterns in your notes. Viewing notes not as mere reminders but as data for pattern recognition dramatically expands your solving repertoire.
Digital vs. Analog Differences
Digital app note features are overwhelmingly more efficient than paper notes. Auto-candidate calculation, auto-removal, and highlighting reduce note management overhead to nearly zero. Paper notes, on the other hand, face spatial constraints (small cells) and update effort. However, paper notes have the advantage that the act of writing itself strengthens memory. Handwriting activates the motor cortex and promotes information retention. Since competitive Sudoku uses paper, paper note management skills remain important.