Pairs
How it works
Two cells in the same row, column or box carry only two candidate digits between them, combined. Whichever cell ends up with which digit, both digits are used up between those two cells — so neither digit can appear anywhere else in that unit.
Try it
Look at row 2. Let's pencil in what's still possible for Row 2, Column 2 and Row 2, Column 5, then see why 1 and 2 are locked into just those two cells.
6
8
4
3
8
9
5
6
9
8
4
6
9
1
7
3
8
2
5
3
6
8
2
9
2
7
8
4
9
5
1
6
3
3
4
2
3
1
7
9
6
3
4
1
5
8
2