Intermediate technique · 03
Two cells reserve the same two candidates.
The idea
A Naked Pair occurs when two cells in a same house collectively contain only same two candidates, allowing to remove those two candidates from all other cells in the house.
The question: Are two cells in this house limited to the same two numbers?
Walkthroughs
It is all about two visible pairs: a pair of numbers, and a pair of cells in the same row, column or box than contain that pair of numbers.
Walkthrough
Two cells r1c2 and r4c2 of column 2 share exact two numbers 5 and 7, no other candidate can fill into these 2 cells, thus numbers 5 and 7 form a Naked Pair. Since these two cells are in the same column 2, therefore number 5 and 7 can be removed from the rest cells of column 2.
What to notice: A Naked Pair is all about two cells in a same house contain exactly two same numbers, and no other cells in the same house can contain those two numbers.
Walkthrough
This example shows a Naked Pair in box 3, where two cells r2c7 and r3c8 contain two numbers 1 and 7 only and no other candidates can go in those two cells, so both numbers 1 and 7 can be removed from rest cells of box 3.
What to notice: You may find that cells r1c7 and r3c7 form a Hidden Pair with two numbers 5 and 8, therefore numbers 1 and 7 can be removed from those two cells. This is an example showing that certain techniques are 'exchangeable', meaning that use one or another will give you the same candicate deduction result.
How it works
Scan a house for cells with two candidates, then test whether two cells together contain exactly the same two numbers. Once two numbers are reserved for two cells, neither number can appear anywhere else in that house.
Look for two cells whose combined candidates are exactly two numbers.
Both cells must belong to the same row, column, or box.
Eliminate those two numbers from every other cell in that house.
A useful reminder
Naked Subset, Pair, Locked 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
Coming soon...
Practice
Once you can explain why the pattern is valid, practice finding it in a few real puzzle positions before moving on.
