|Hidden Doubles (Hidden Twins, Hidden Pairs)
A Hidden Double occurs when a pair of candidates is confined to two cells in any house
When that occurs, the remaining candidates in those two cell can be eliminated.
Finding hidden doubles can be very difficult. (click here to see what I mean).
The pair 16 is confined to two cells in column 4 (and also box 5 in this case).
Hidden doubles will eliminate the other candidates in those two cells.
(to many sudoku fans, it's more obvious to see the naked quad in column 4 (4789). than
to see the hidden double. Finding the naked quads will cause the same eliminations.)