How to Use Candidates in Sudoku

Candidates, often called pencil marks, are temporary possible numbers for a cell. Used well, they make hard puzzles manageable. Used badly, they create noise and slow you down.

What Candidates Are

If a cell cannot be solved immediately, write only numbers that do not violate row, column, or 3x3 box rules. That small set is the candidate list for that cell.

When to Use Candidates

  • Use candidates when singles are exhausted.
  • Use them in constrained regions first, not everywhere.
  • Expand gradually only when needed.

This targeted approach keeps your board readable.

How to Keep Candidate Notes Clean

Remove outdated values fast

Every placement affects related row/column/box cells. Remove invalid candidates immediately.

Avoid over-annotation

Writing every candidate in every empty cell early is rarely efficient.

Use consistent notation

Consistent order (for example 1-9 left-to-right) helps your eyes spot patterns faster.

Patterns You Can Spot with Candidates

  • Naked pair: two cells share the same two candidates.
  • Hidden single: a candidate appears once in a unit.
  • Locked candidate: candidate confined to one line within a box.

These patterns are central to solving medium and hard boards without guessing.

FAQ

Should beginners use candidates immediately?

Not always. First learn singles and scanning discipline. Then add candidates for harder boards.

How many candidates are too many?

If you cannot scan quickly, you have too many. Reduce to constrained areas first.

Where can I practice candidate techniques?

Use Sudoku One9x and focus one candidate pattern per session.

30-Minute Candidate Drill Plan

Run this drill three times per week: pick one medium puzzle and stop after each major placement to clean candidates in all affected units. Your goal is not completion speed; your goal is note quality. At the end, count how many stale candidates remained. Over time that number should drop.

  • Session A: focus on candidate cleanup speed.
  • Session B: focus on spotting pairs from notes.
  • Session C: focus on locked-candidate opportunities.

Key Result to Track

Track "candidate errors per puzzle" and "time spent stuck with full notes." Lower numbers mean your notes are becoming decision tools instead of clutter. Practice this pattern in Sudoku One9x.