Locked candidates
How it works
Pointing pair or triple. Look at one 3×3 box: if every remaining cell that could hold a digit lines up in a single row or column, that digit must end up somewhere in that line inside the box — so it can be eliminated from the rest of that row or column outside the box.
Try it
Look at box 3. Let's pencil in what's still possible there, then see why digit 3 is forced into just one line.
9
8
5
2
7
9
3
2
6
4
1
2
7
8
5
1
3
6
7
9
3
9
5
5
9
9
2
8
1
3
5
7
6
4
3
2
9
5
1
8