He tried to proceed. He needed a '4' in the second row. He calculated the possibilities. Box 2 blocks the 4 here... Row 2 blocks it there... He wrote a small '4' in the margin.
Instructions:
: This strategy links cells that have only two candidates. By following the "chain" across the board, you can eliminate a candidate from a cell that "sees" both the start and the end of the chain. Why "129" Matters for Your Brain sudoku 129