### Subset2, or Pair, or Naked Twins, or Naked Pairs - Advanced Solving Sudoku Technique

Subset2 by createclassicsudoku.com

Naked Twins by humage.com

Naked Pair by hodoku

Subset2, or Pair, or Naked Pair, or Naked Twins:

If two blank squares in a same row/column/box has same two candidate numbers, then other empty squares in this row/column/box can remove these two numbers from their candidate numbers.

Candidate Numbers (or Candidate Values) of an blank/empty square is a list of 'possible values' or candidates for this blank/empty cell.

Below is an example of using Subset2(a.k.a. Pair, or Naked Pair, or Naked Twins) to refine the candidate values of R7C1, R7C6, and R7C7. Blank/Empty Squares are marked with candidate values in red.

Medium Sudoku on 03/13/2018 For the seventh row, R7C3 and R7C4 have same two candidate values {1, 4}. Therefore, it is safe to remove numbers 1 and 4 from the candidate numbers from other blank/empty squares in the seventh row. After we remove numbers 1 and 4 from R7C7's candidate values, R7C7 has one and only one candidate value (number 8) left. Therefore, R7C7 must be filled with number 8. For convenience, our Sudoku solver names 3 different categories of Subset2.

Subset2Row:

an untaken number in a row only show up in one empty square as its candidate values, then this empty square must take the number.

Subset2Col:

an untaken number in a column only show up in one empty square as its candidate values, then this empty square must take the number.

Subset2Box:

an untaken number in a 3×3 box only show up in one empty square as its candidate values, then this empty square must take the number.

### How to find them?

• Use basic methods to solve the Sudoku until no further blank/empty square can be inferred to be a number using basic methods (Sole Candidate and Hidden Single).
• Write down the candidate values for every blank/empty square.
• Check every row/column/box to see if there are two blank/empty squares have same two candidate values.
• When there are two blank/empty squares with the same two candidate values, remove these two values from other blank/empty squares in the same row/column/box.

##### Solving Sudoku Methods Explained

This article answers questions: What are commonly used solving sudoku methods? What solving Sudoku methods do we use? In what sequences?

##### Basic Solving Sudoku Method: One Choice

a.k.a Sole Candidate, Only Candidate, or Naked Single

##### Basic Solving Sudoku Method: Elimination

a.k.a Hidden Single, Loner

##### Advanced Solving Sudoku Method: Subset2

a.k.a Pair, Naked Pair, or Naked Twins

##### Advanced Solving Sudoku Method: SubsetPosition2

a.k.a Hidden Pair or Hidden Twins

Last updated 3 mins ago

##### Advanced Solving Sudoku Method: Interaction

a.k.a Box/Row Claim or Intersection , or Locked Candidates

##### Solving Sudoku Methods Explained

This article answers questions: What are commonly used solving sudoku methods? What solving Sudoku methods do we use? In what sequences?