Sudoku-YNext steps
Naked triples and quads: count the candidate union
Instead of looking only for identical notes, count how many distinct digits a group of cells can use.
Practise on a board ↓LOOK · TRY · UNDERSTAND
Try it and see why
Three cells, different notes
The empty cells in row D have the complete notes shown. Large digits are already placed.
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
Use the three evidence cells to remove candidates from D9.
Select a cell, then tap a candidate below.
Removed 0/2
See the reasoning
D1, D4 and D7 together allow only 2, 5, 8. Three cells use those three digits, even with different notes. D9 is left with 6.
Digit set 1/9 · A different digit set in the same pattern, not a new full puzzle.
Read the source principles: HoDoKu — Naked Subsets
Count distinct digits
Three cells in one row, column or box whose complete candidate union has three digits reserve those digits. Remove them from other cells in that unit. A naked quad applies the same constraint to four cells and four digits.
HoDoKu — Naked Subsets · SudokuWiki — Naked Candidates
Crossed-out digits are eliminations. Check the full conditions in the worked example below.
Different notes, one triple
D1={2,5}, D4={5,8}, D7={2,8} have union {2,5,8}. These digits are reserved for the three cells. D9={2,6,8} therefore keeps only 6, even without deciding D1 first.
Try before revealing
Does the same triple work if D4 becomes {5,8,9}?
Show explanation
No. The union is now four digits, {2,5,8,9}, across three cells. That does not establish this naked triple.
A quad in row E
E1={1,3}, E3={3,5}, E5={5,7}, E7={1,7} reserve four digits in four cells. Remove 1 and 7 from E9={1,2,7}. Count distinct digits, not occurrences, and ensure all source cells belong to one unit.
On the live board
The scanner supports triples and quads. It identifies the unit, displays each source cell’s candidates and temporarily crosses out the removable candidates. Reading an example does not change your saved notes.
References and further reading
Read the source explanations and compare them with our examples. Links are references, not endorsements or claims of affiliation.