Sudoku-YFoundations
Pencil notes: candidates are not confirmed answers
Small numbers are possibilities, not commitments. Keeping that distinction clear makes it easier to justify a move and revisit an earlier elimination.
Practise on a board ↓LOOK · TRY · UNDERSTAND
Try it and see why
One missing note changes things
The shown cells have complete notes. Check D1 and D4.
Our own partial teaching board, not a full puzzle. Large digits are placed values; small digits are candidates. Dots mean the cell is not described here. Complete candidate lists are part of the stated scenario, not calculated from the dots.
Green outline = evidence · Purple “try” = an assumption · Crossed out = removed candidate
Can this Naked Pair remove 8 from D7?
See the reasoning
D1 and D4 use 2, 8 in some order in row D. Other cells in the row cannot use either digit.
Digit set 1/9 · A different digit set in the same pattern, not a new full puzzle.
Read the source principles: HoDoKu — Naked Subsets
Start with the three related units
For an empty cell, collect the placed digits in its row, column and 3 × 3 box. Remove them from 1–9 because each unit must contain each digit once. What remains is a basic candidate list; further reasoning may reduce it.
Work through a small example
Suppose the related units together contain {1, 2, 4, 5, 7, 8, 9}. The target has candidates {3, 6}. This alone does not establish which is correct. Look for another justified move.
Try before revealing
A confirmed 6 is later placed in the same column. What remains in the target?
Show explanation
Only 3. The placed 6 rules out 6 in the target; the stated conditions do not rule out 3.
Avoid these note-taking mistakes
A neighboring note is not a placed digit, so its mere presence does not eliminate your candidate. Update notes after changing answers. If a cell has no candidates, review your entries and earlier deductions instead of guessing a number to fill the gap.
In Sudoku-Y
Notes (+) enables pencil entries. Tap again to eliminate a candidate and again to restore it. Fill all candidates applies the basic filters across the board for free; it does not establish the final answers. Hiding eliminated notes changes their visibility, not their history.
References and further reading
Read the source explanations and compare them with our examples. Links are references, not endorsements or claims of affiliation.