When three numbers can only go in the same three squares of a row, column or box, those squares hold them. Every other option in the three squares goes.
123456789
123456789
3
27
249
59
6
479
257
8
17
19
6
8
2
3
4
157
17
8
57
4
59
1
79
3
57
6
2
1789
19
3
4
89
6
17
5
145
3
2
7
6
8
9
14
4679
6789
49
1
5
89
2
3
47
4
8
7
9
2
6
3
59
3
7
6
8
1
4
2
169
69
9
4
3
5
7
8
The triple's three squaresLoses a pencil mark
The other side of the box
The box has seven empty squares. The other four, row 4, column 3, row 5, column 1, row 5, column 2 and row 6, column 3, hold 1, 4, 5 and 9 between them: a hidden set always comes with a naked one in the group's other squares.
How to spot one
Pick a group, list its missing numbers and count the places for each. Three numbers that share the same three places make a hidden triple.
What it gives you
Cleaner squares. Here two of the three come down to two options each.
Practise Hidden triple
Boards that need it, with the owl's hints when you're stuck.