sudokUI › Sudoku techniques › Hidden Quadruple
Hidden Quadruple in sudoku
flip side of a naked subset · in a line: Jellyfish sideways · how it fits
Four digits, all still missing from a unit, whose places in that unit fall within the same four cells, though a digit need not appear in all four.
Why it works
The unit needs all four digits in four different cells, and only these four cells can take them, so the digits fill all four cells. Remove all candidates except those four digits from the four cells.
How to spot it
In a unit with many empty cells, find digits with at most four places, then check whether four of them fit the same four cells.
Worked example
Hidden Quadruple: in box 5, digits 1/2/5/8 fit only in r4c5, r4c6, r5c5, r5c6, so all other candidates there can be removed.
Practise it on a real puzzle. sudokUI generates a puzzle that needs Hidden 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 Hidden Quadruple, the puzzle's difficulty rating grows by 150. Its class is Hard. It is needed in 1 in 12,000 puzzles that sudokUI generates. How the rating works.
More subsets
Naked Pair · Naked Triple · Hidden Pair · Hidden Triple · Naked Quadruple