The Hidden Pairs Technique in Sudoku
Spotting Hidden Pairs
Hidden pairs become evident when noted candidates (pencil mark) are made alive on the grid.
For example, when marking candidate 4 in a filled grid and 8 as candidates enabled, cell R3C4 becomes R3C6, making it intuitive to assume no further
fills are applicable for those cells.
Hidden vs Naked subsets
Tracking multiple candidates throughout cells can be complicated needing stronger pattern recognition as in larger hidden subsets (triples, quads).
+ Personally dependent strategies leave many with marking all possibilities and seeing groups, thus prefer naked subsets (visible groups of
candidates).
How Hidden Pairs Work
A hidden pair occurs within specific house living arrangements that two candidates, select must exclusively
occupy enabling all else to be removed, rendering the switch resets the house switches.
Example 1 (Column 9):
Candidates 1 and 9 are restricted to R5C9 and R7C9. Even though 1 and 9 are available in Rows 5/7 and Blocks 6/9, their positioning is only sensible
within these two cells in Column 9. This clears out irrelevant candidates such as 6 from R5C9.
Example 2 (Row 1/Block 1):
Cells R1C1-2 hide a pair with 4 and 7 concealed behind 10 other candidates. All other candidates aside from 4 and 7 can be eliminated in these
cells.
After studying this example, explore Naked Pair and Hidden Triple to compare how visible and concealed pairs lead to candidate elimination.