According to Sudoku rules, each number should appear at list once in each box, row, and column. Hidden Single
is a number that appears as a possible candidate only in one cell for the whole box, row, or column. In this case this number should go into the cell, since there are no other candidates. A picture on the left shows an example of Hidden Single
in a box. The number '5'
(marked green) in box (1*1) appears in the list of possible candidates only in cell A3. Number 5 is the solution for the cell A3.