The Locked Pair Technique in Sudoku

Locked Pair

When two cells forming a _Naked Pair_ (two identical candidates confined to two cells) are located in two overlapping houses (e.g., a row and a

block, or a column and a block), this is termed a Locked Pair. Candidates can be eliminated from both houses.

Example 1

Cells R3C7 and R3C9 form a Locked Pair in Row 3 (R3) and Block 3 (B3). This eliminates candidate 2 from R3C3 (within the row) and R1C9 (within the

block).

Example 2

A Locked Pair in Row 8 (R8) and Block 8 (B8) removes 18 candidates across both houses.

After this example, take a look at Locked Triple and the broader Locked Candidates Technique to understand how candidate restrictions scale in complexity.