Sudoku Strategies: Hidden Singles
Understanding the Hidden Single Technique
The Hidden Single is one of the most basic and frequently used Sudoku strategies. It is often easier to spot with pencil marks or by using a method called "cross-hatching".
How to identify a Hidden Single:
- Focus on one house: Pick a single row, column, or 3x3 block.
- Focus on one digit: Choose a number from 1 to 9 that is not yet placed in that house.
- Scan the house: Look at all the empty cells in that house and determine where that specific digit *could* go based on existing numbers in intersecting rows/columns/blocks.
- Find the only spot: If there is only one cell where that digit can possibly be placed within that house, you have found a Hidden Single. You can safely place that digit in that cell.
Example:
Consider the cell in the illustration. Cell "E8" (Row "E" Column "8") is the only cell in that house (a house is a row, column, or block) where 3 can exist as a possibility. Similarly, cell "E1" is the only place in that house where a 3 could exist. Notice that other 3 candidates can exist in the same block or column. The only requirement for a hidden single is for the cell under consideration to hold a unique candidate for 1 of the 3 houses in which the cell exists.
Mastering this technique is essential for solving most easy and medium Sudoku puzzles.
