Triples
How it works
Three digits, within one row, column or box, only fit into the same three cells — even though those cells may still show other candidates too. Since all three digits have to go somewhere in that unit, and only these three cells can take them, every other candidate can be cleared from those three cells.
Try it
Look at box 1. Let's pencil in what's still possible for Row 2, Column 1, Row 2, Column 3, and Row 3, Column 3, then see why 4, 5, and 6 are hiding there.
6
5
4
9
7
2
8
4
7
3
8
2
4
6
4
3
6
5
3
8
8
4
3
2
6
5
6
4
3
8
2
2
5
3
7
4