Lines and boxesBasic boards
Locked candidates
If a box's places for a number all lie in one of its rows, the box will put that number in that row. The rest of the row can't have it.
Where the box's 5 can goLoses a pencil mark
Two directions
It works from box to line, as here, and from line to box: if a row's 5 can only be in one box, the rest of that box can't be 5.
How to spot one
When you search a box for a number and its places line up in a row or a column, you've found one.
What it gives you
Only removals. But clearing a row often leaves another box with one place for the number: a hidden single.
Practise Locked candidates
Boards that need it, with the owl's hints when you're stuck.