Sudoku Tutorial
Naked Quads
When a house has four cells which contain only combinations of the
same four numbers, those are Naked Quads.  

In column 3, the yellow cells all contain only 3,4,5, and 8.  That means that the 3458
cannot exist in any other cells of that house (column 3).

Column 3 also has a hidden quad (click here to see it)


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