Sudoku Tutorial
Hidden Quads
A hidden quad occurs when four numbers (a,b,c, and d)  are confined to four cells in a house.  
Also one or more of these cells must contain other candidates other than  a,b,c, and d 
(otherwise it would be a naked quad).  

In column 3, candidates 1267 are confined to the four yellow cells.  Any other candidates in
those cells can be eliminated.  

Column 3 also has naked quads (3458).  This will perform the same eliminations as the hidden quad
(click here to see it)


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