Shaded squares must contain even numbers; shaded circles must contain odd numbers. Advanced Solving Strategies
If you notice that two squares within the same row, column, or box contain the exact same pair of candidate numbers (for example, both cells only have the options [2, 7] ), you have found a Naked Pair. While you do not know which square is 2 and which is 7, you know with absolute certainty that those two numbers must occupy those two cells. Therefore, you can safely eliminate 2 and 7 from the candidate lists of all other cells in that same row, column, or box. The same logic applies to Naked Triples (three cells sharing three candidates). Pointing Pairs sudoku 129
The core objective of the puzzle is simple but rigid. Every row, every column, and every 3x3 box must contain the numbers 1 through 9 exactly once. No repeats are allowed in any line or block. Essential Solving Techniques for Beginners Shaded squares must contain even numbers; shaded circles