Intermediate technique · 04
Two numbers are hiding in two cells of a house.
The idea
Those two cells may contain other candidates, but the two hidden numbers are restricted to those two cells. The extra candidates can therefore be removed from those two cells.
The question: Which two numbers are forced to share two cells in this house?
Walkthroughs
Hidden Pair only appears in a house with more than two empty cells.
Walkthrough
In this example, in either box 5 or column 6, numbers 1 and 7 can only appear in cells r4c6 and r5c6, therefore number 1 and 7 form a Hidden Pair, and rest of numbers 4, 5, and 9 can be removed from those two cells.
What to notice: The two cells holding Hidden Pair could be part of one or two houses; it is hidden because the two cells may contain extra candidates, and the extra candidates can be removed from those two cells.
Walkthrough
This example shows that in column 6, numbers 5 and 9 can only appear in cells r4c6 and r9c6, therefore number 5 and 9 form a Hidden Pair, and rest of numbers 7 and 8 can be removed from those two cells.
What to notice: The pair is hidden because the two cells may contain extra candidates. The restriction belongs to the numbers: they have nowhere else to go in column 6.
How it works
The pair is hidden because the two cells may contain extra candidates. The restriction belongs to the numbers: they have nowhere else to go in the house. You discover it by following the numbers, not by looking for cells that already contain only two candidates.
Choose two numbers, and track where each number can still appear within the house.
If both numbers are confined to two cells in this house, the hidden pair is present.
Remove every other candidate from those two cells.
A useful reminder
Hidden Subset, Hidden Pair are related terms you may encounter while learning this technique. The important part is understanding the constraint and why the elimination follows from it.
Watch & learn
Comming soon...
Practice
Once you can explain why the pattern is valid, practice finding it in a few real puzzle positions before moving on.
