Sudoku Strategies: Naked Doubles

Naked Doubles in Sudoku

A naked double (also known as a naked pair) is a Sudoku solving technique that identifies two cells within the same row, column, or 3x3 block that contain only the exact same two candidate numbers.

Explanation

When you find two cells in a single house (row, column, or block) that are restricted to only two identical possible numbers, you can be certain that those two numbers must fill those two specific cells in the final solution.

• The Key Logic: Even though you don't yet know which of the two numbers goes in which cell, you know for a fact that the pair of numbers will be used by those two cells exclusively within that unit.
• The Actionable Step: Because the numbers are "reserved" for those two spots, you can eliminate those two candidates from all other cells in that same unit (row, column, or block).

Example

In row 8 in the illustration you will notice that cells "B8" and "F8" both contain (exclusively) the same two candidates- in this case candidates 1 and 2. As you can see in the illustration, this leads to several eliminations in cells "G8","H8", and "I8" because candidates 1 and 2 cannot be the solution for those cells.

Remember, the logic works for rows, blocks (3x3 squares), or rows.