sudokUI › Sudoku techniques › Naked Quadruple
Naked Quadruple in sudoku
flip side of a hidden subset · in a line: Jellyfish sideways · how it fits
Four empty cells in the same unit whose candidates, taken together, are exactly four digits, though no single cell needs to hold all four.
Why it works
Four cells in one unit need four different digits, and only those four are available, so the cells use up all of them. Remove the four digits from all other cells of that unit.
How to spot it
In a unit with many empty cells, look for four cells whose candidates never stray outside the same four digits.
Worked example
Naked Quadruple: cells r5c4, r5c5, r5c6, r5c9 contain only 5/6/8/9, removing those digits from the cells they all see. So 9 is removed from r5c3, r5c8.
Practise it on a real puzzle. sudokUI generates a puzzle that needs Naked Quadruple and takes you straight to the position where it applies. Hints draw the pattern on the board and explain the step.
In a puzzle's rating
Each time the solver needs Naked Quadruple, the puzzle's difficulty rating grows by 120. Its class is Hard. It is needed in 1 in 840 puzzles that sudokUI generates. How the rating works.
More subsets
Naked Pair · Naked Triple · Hidden Pair · Hidden Triple · Hidden Quadruple