I think I am missing something in the instance below, since I could solve it only with a long-winded chain of deductions starting with the assumption that (coordinates are (row,column)):
8 is in (8,7),
and continuing as follows:
=> 8 is in (7,6) => 9 is in (1,4) => 9 is in (5,5) => 9 is in (4,1) => 9 is in (7,3) => 3 is in (9,2) => 3 is in (8,7)
I just started learning Sudoku and it never happened before to have to do this, so maybe I am missing something, or I am not aware of a simple(r) technique applicable in this instance.
Coordinates are (row,column), starting with the upper left square which is (1,1);
Candidates for a number in a box are filled in only if there are at most two of them. With the exception of 3 and 8 in the bottom-center box, that are filled in even if there are 3 candidates.