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Let's Make Number Place (Elementary course)
Strategy Guide
Number Place Strategy Guide
Hidden Pair
When two numbers can only be placed in two specific squares in a given group (horizontal line, vertical line, and block), each of those numbers must go into one of the two squares and you may therefore remove any other candidate numbers from the two squares.
Example 1
Watching the numbers 3 and 4, you can see that these numbers cannot be entered into any but the two blue squares of the upper left block. Therefore, one of the blue squares must absolutely be 3 and the other must absolutely be 4 and any other candidate numbers for the two squares may be deleted.
Example 2
Pay attention to the second vertical line from the left.
Numbers 1 and 2 cannot appear anywhere else in the upper left block and are crossed out from two other horizontal lines. Therefore 1 and 2 must appear in the two blue squares. Since 1 and 2 must appear in these squares, one square must absolutely be 1 and one square must absolutely be 2.
Why Are They Called "Hidden Pairs"?
We'll explain using Example 1.
The small numbers in the image above indicate the possible numbers for each open square of the upper left block of Example 1. Numbers 3-9 can be put into either of the two blue squares.
However, the truth is that the two blue squares can only be 3 or 4, and therefore numbers 5-9 cannot appear in them.
In other words, ultimately the 3/4 pair will remain in the two blue squares. Since the other candidate numbers mask the existence of this 3/4 pair, it is called a "hidden" pair.
"Hidden Pair" is a term for the opposite of a naked pair.