Intermediate technique · 02
Use a box-row or box-column relationship to eliminate candidates.
The idea
Locked Candidates occurs when all possible locations for a number in one house are confined to another house. That relationship lets you eliminate the number from the rest of the intersecting house.
There are two directions: Pointing starts in a box and eliminates from a row or column; Claiming starts in a row or column and eliminates from a box.
The question: Is this number locked into one row or column within a box(Pointing) — or into one box within a row or column(Claiming)?
Walkthroughs
Locked Candidate is all about intersection of 2 houses.
Walkthrough
In this example, box 3 has number 8 locked to cell r1c7 and r3c7, so in its intersecting house, which is column 7 in this case, the rest cells cannot have number 8. This removes number 8 from cell r4c7, r5c7 and r6c7.
What to notice: Pick a number, then scan boxes to find out if the number is locked to a particular row or column.
Walkthrough
Look at this same example from a different angle: column 9 has number 8 locked to cell r4c9 and r6c9, and these two cells are also part of box 6; so in this intersecting house, the rest cells cannot have number 8. This removes number 8 from cell r4c7, r5c7 and r6c7, exactly the same deduction as in previous example.
What to notice: Pick a number, then scan rows or columns to find out if the number is locked to a particular box. Also notice that certain techniques are "exchangeable", meaning one or another gives the same deduction result. We will see this often later in other examples.
How it works
A candidate does not need to be solved to give you information. If every possible location for a number inside one house lies in the same intersecting line, the rest of cells in the intersecting house cannot contain that number.
Work with one candidate at a time.
For Pointing, check a box; for Claiming, check a row or column.
Remove the candidate from the other cells in the intersecting house.
A useful reminder
Pointing and Claiming are two candidate reduction techniques which remove a same set of candidates from other cells but from different angles.
Intersection, Pointing, Claiming, Locked Candidate Type 1, Locked Candidate Type 2 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.
