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Let's Make Number Place (Elementary course)
Strategy Guide
Number Place Strategy Guide
Naked Pair
When the same two numbers can only be inserted in two places within the same vertical line, horizontal line, and block (altogether called a group), you can remove the two numbers from the candidate numbers of the other squares in the group.
Example 1
The two blue squares can only be 8 or 9 since the numbers 1-7 appear in either the vertical line, horizontal line, or same block as the two blue squares.
Since the numbers 8 and 9 must go into these two squares, the four other magenta squares cannot possibly be 8 or 9.
Example 2
The same strategy may be used for the nine squares in the horizontal line at the top of this game board.
Since the numbers 1-7 are taken in either the vertical or horizontal lines around the blue squares, the blue squares must be either 8 or 9.
As the numbers 8 and 9 must go into the two blue squares, the remaining four magenta squares of the horizontal line cannot be 8 or 9.
Why Are They Called "Naked Pairs"?
I'll explain using Example 1.
If you delete the numbers used in the vertical lines, horizontal lines, and block from the possible candidate numbers of each open square, you'll get the image above.
The only candidate numbers for both of the blue squares are 8 and 9. These types of squares are called "naked" since no other candidate numbers but this pair exist for the two squares.
Of course, this is in relation to the term "Hidden" from the strategy "Hidden Pair."
Since 8 and 9 can only appear in the two blue squares, you can delete 8 and 9 from the possible candidates of the other white squares.