“Hidden Pairs” Sudoku technique explanation:

two sudoku boards demonstrating the hidden pairs technique

“Hidden pairs” happens when a pair of numbers appears in exactly two cells in a house (a house is a row, column, or a block), but those two numbers aren't the only ones in their cell.

See the example:

Take a look at the column marked in pink, it has two cells marked in blue.

sudoku suggesting a path to hidden pairs

we can see that there are 1 and 9 in both cells in the same column they are marked in yellow.

sudoku board demonstrating the hidden pairs technique

Those are our hidden pairs.

This is why it’s hidden pairs and not a “Naked Pair”. essentially, in other words, a hidden pair is a hidden “Naked Pair”.

When you spot a hidden pair you should take out any other candidates from the two cells.