Naked Singles
Remember buddy cells?
Buddy cells are cells that are in the same house (same row, same column or same box).
Have a look at R5C8 in this puzzle, its buddy cells are highlighted in grey.
Let's examine the possible values of R5C8 - these are called candidates.
To begin with, R5C8 can be any digit between '1' and '9'.
So let's keep a note of this on the side.
The notes in the image are called pencil marks and we'll discuss this in detail on the next tutorial
Looking at row number 5 - we have '2', '9' and '3' already in use.
Being on the same row as R5C8, this means that R5C8 cannot contain '2', '3' or '9'.
The basic rule says that a digit can appear only once on the same row.
So let's eliminate '2', '3' and '9' from our candidate list.
Looking at R5C8's box - we have '1', '5', '6' and '7' used already.
The basic rule says that a digit can appear only once on the same box,
So let's eliminate '1', '5', '6' and '7' from our candidate list for R5C8.
Last, we'll examine R5C8's column - it has '2' and '4'.
Once again, a digit can appear only once on the same row so we'll eliminate '2' and '4' from the candidate list for R5C8 ('2' was already eliminated).
That's it, what do we have left on our candidate list for R5C8?
There's only one possibility left and it's an '8'.
So R5C8 must be an '8'.
Here's another example of a naked single.
Can you see the answer for R4C6?
Once again it's an '8', because its buddy cells contain every digit from '1' through '9', except for '8'.
