Quadruple Sudoku Explained in One Short Example
A circle showing 1 and 3 sits where four cells meet. The 1 and the 3 both appear in those cells, and the other two cells are free.
Transcript
The rule
A quadruple is a small circle on the point where four cells meet. Every digit in the circle must appear in at least one of those four cells.
The circle doesn't say which cell holds which digit. Cells that don't get a listed digit can hold anything the other rules allow, and a circle with four digits fills all four cells.
Worth memorizing
The circle doesn’t say where, only that each listed digit is somewhere in the four cells.
Four listed digits fix the four cells’ contents completely.
A digit listed twice appears twice, so the two copies must be diagonal to each other.
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