Indirect solving of a Sudoku : examination of the "1" candidate. Allowed pairs have been marked.

• "1" is found in cell "Aa", preventing another "1" from being in line "A" and column "a" (yellow).
• In the upper central block, "1" can only be in column "d" (green).
• Hence, in the lower central block, "1" can only be in column "e" (green).
• Hence, in the central bloc, "1" can only be in column "f" (blue).
• In the central block, "1" is now restricted to lines "E&F" (blue).
• In the left central block, "1" has been restricted to lines "E&F" (green).
• Hence, in the right central block, "1" is restricted to line "D" (blue).
• In the right central block, "1" is now restricted to columns "h&j" (blue).
• In the right upper block, "1" was also restricted to columns "h&j" (green).
• Hence, in the right lower block, "1" is prevented from being in columns "h&j" (yellow).
• Therefore, value of the two cells marked with pairs "12" and "13" is "2" and "3".