Unique Candidate Technique Explained

This is another simple technique - sometimes also called 'Hidden Single'.

Basically,

If a cell is the only one in a 3x3 block , column or row that can take a particular candidate (number), then it must be that candidate.

This follows on from the general rules that each 3x3 block, column and row must have the numbers 1-9.

Check out this example:

The middle 3x3 block must have a 9, but it can't be in the top 2 rows. That leaves the only other vacant cell - that must be where the 9 goes.

Cheers,

Gath