Technique
Hidden single Sudoku technique
A hidden single is a digit that can go in only one square within a row, column, or 3 by 3 box, even if that square has other candidates.
What it means
Instead of asking what can fit in one square, ask where a specific digit can fit inside one unit. If a 6 has only one possible square in a row, that square must be 6.
Row, column, and box examples
Row example: if every square in row 4 blocks 8 except column 1, then row 4 column 1 is 8.
Column example: if a 3 can fit only once in column 7, place 3 in that square.
Box example: if the top-left 3 by 3 box has only one possible spot for 5, that spot is a hidden single.
How to spot it
Pick one digit and scan a row, column, or box. Cross out squares that already see that digit. If one square remains, you found a hidden single.
Notes help because they make the possible positions for a digit visible. Keep notes tidy so the hidden single is not buried.