What are candidates in Sudoku?
Candidates are the digits that are still legal for an empty cell after you exclude numbers already present in its row, column, and 3×3 box. Pencil marks are simply a way to record those candidates so you can compare patterns across the grid.
A candidate is not a guess. It means “this digit has not been ruled out yet”. Good solving comes from removing candidates with valid logic until a forced placement appears.
How to build a clean candidate map
For each unresolved cell, start with digits 1–9 and remove every digit already used in the cell’s row, column, or box. The remaining set is the candidate list for that cell.
You do not always need full-grid notes on Easy puzzles. On harder puzzles, however, a consistent candidate map makes intermediate patterns much easier to verify and reduces the risk of eliminating a digit from the wrong place.
- Update notes immediately after every confirmed placement.
- Never keep a candidate that conflicts with a row, column, or box.
- Use the same notation consistently so patterns are easy to compare.
Stale pencil marks create false techniques
If you place a digit but forget to remove that digit from peer cells, later candidate patterns may be fake. A supposed pair or X-Wing can disappear as soon as the notes are corrected.
When a technique produces several eliminations, pause and rescan the affected units before looking for another advanced pattern. Clean notes are more valuable than a crowded grid full of outdated marks.
Which techniques use candidates?
Hidden Singles use candidate positions within a unit. Locked Candidates compare candidate positions across a box and a row or column. Pairs reserve two digits in two cells or two positions. X-Wing extends the same position logic across two rows and two columns.
That progression is why candidate accuracy sits in the middle of the learning path: it connects beginner Singles to the structures used on Hard, Expert, and Master puzzles.
Apply it immediately on a real puzzle
A technique becomes automatic only after you use it across many different boards. Play at a manageable level, prioritize certain deductions, and review the step that caused you to get stuck.