Intermediate technique · 01
Track what is possible before you decide what is true.
The idea
Candidates record the numbers that are still possible in an unsolved cell. They turn the empty grid into a map of constraints you can reason from. Those notes are the raw material for every subset, locked-candidate deduction and any other techniques that follows.
The question: Which candidates are still genuinely possible here?
Walkthroughs
What possible candidates a cell can have is constrained by the row, the column and the box that contain the cell.
Walkthrough
The example below shows a grid with initial candidate fill-out for all empty cells, no any candidate duction has been made.
What to notice: Candidates make eliminations visible. Each placed number or logical deduction removes possibilities from cells that can see it, and accurate notes let you notice patterns that are otherwise easy to miss.
How it works
Candidates make eliminations visible. Each placed number or logical deduction removes possibilities from cells that can see it. Accurate notes let you notice patterns that are otherwise easy to miss. If a candidate is wrong or missing, later techniques can appear to work when they do not.
Work with one number or one house one by one. Note only numbers allowed by the cell’s row, column, and box.
Once candidates are visible, look for restrictions that can eliminate candidates elsewhere.
When a number is placed or eliminated, remove that candidate everywhere required.
A useful reminder
Pencil marks, candidates, notes, potentials, possibilities 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.
