Candidates are the possible digits that can be placed in each empty cell. Tracking them is essential for intermediate and advanced solving.
In Sudoku, a candidate is any digit from 1 through 9 that could legally be placed in a specific cell. A digit is a valid candidate for a cell if it does not already appear in the same row, column, or box. As you fill in cells and apply solving techniques, the number of candidates for each remaining cell decreases.
When a cell has been reduced to a single candidate, that digit must go in that cell. This is the most basic form of solving, known as a naked single. Understanding candidates is essential for progressing beyond the simplest puzzles. For a broader overview of solving approaches, see our Sudoku tips guide.
The most common way to track candidates is through pencil marks, small notations written in each cell listing its remaining possibilities. On paper, solvers typically write small digits in the corners or center of each cell. Digital Sudoku apps usually provide a dedicated pencil mark mode for this purpose.
To determine the initial candidates for a cell, check which digits from 1 through 9 are already present in that cell's row, column, and box. Any digit not yet present in all three groups is a valid candidate. For example, if a cell's row contains 1, 3, and 5, its column contains 2, 3, and 7, and its box contains 1, 4, and 9, the candidates for that cell would be 6 and 8.
Nearly every intermediate and advanced Sudoku strategy relies on candidates. Techniques like naked pairs look for cells that share the same two candidates, allowing eliminations from other cells in the same house. Hidden singles find candidates that appear in only one cell within a house.
The key insight is that every solving technique is ultimately about reducing candidates. Whether you are using basic crosshatching or advanced X-Wing patterns, the goal is always the same: eliminate candidates that cannot be correct until each cell has exactly one possibility remaining.
Explore more Sudoku terminology in the complete glossary.
A candidate is any digit that could potentially be placed in a given cell based on the current state of the puzzle. As you eliminate impossible options through solving, the list of candidates for each cell shrinks until only one remains.
Candidates and pencil marks are closely related but not identical. Candidates are the theoretically possible digits for a cell based on the puzzle's constraints. Pencil marks are the notation you write to track those candidates. You can have candidates without writing pencil marks, but pencil marks represent candidates.
For easy puzzles, you can often solve without explicitly tracking candidates. However, for medium and harder puzzles, tracking candidates becomes essential. Most intermediate and advanced strategies like naked pairs and hidden singles require knowing which candidates remain in each cell.
A cell can have anywhere from 1 to 9 candidates. A cell with only one candidate is immediately solvable. At the start of a hard puzzle, many cells may have 4 or more candidates, which are gradually reduced through elimination techniques.
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