Hidden Single
(Unique Candidate)
easy
The pattern I look for most frequently when starting a puzzle is called a hidden single. In this case you can rule out a number from all but one of the squares in a container. Here’s an example.
You’ll see that in the center block, the question mark is in the only square that can hold a 2, due to the presence of 2s in the adjacent columns. This configuration makes a hidden single in the center block, and also in column 5.
The following intermediate puzzle from my collection contains four hidden singles, three of which I’ve highlighted.
What numbers must they be?
Can you find the other one?