Lets look at a new puzzle. This one is classified a standard by Simple Sudoku:
Now when we apply the first 3 rules to the puzzle we get to this:
So where do we go from here. We need a new rule.
Rule #4: If there are only two or three of the same candidate in the same box and in the same row or column then the candidate will be in that box and not in the rest of the row or column.
Lets look at an example:
We can see here with the 4's that in the lower left hand box there are two 4's in the same column. That means that one or the other has to be a 4 so no where else in that column can there be a 4 so we can exclude them. Like so:
Well this helps some but we have not gotten to the end chain yet so we keep going.
We can see a Rule #4 here too and it starts a small chain but do not finish the puzzle. So we move on.
Nothing that solves the puzzle right now but we are getting closer.
As we go thought the numbers again we will fine a Rule #2 in the 1's and 6's. As it turns out it was the small 6's chain that we need to solve the puzzle. What we see when we get to the 9's again is this.
In the bottom right hand box we see a Rule #4 When we exclude the 9 candidates because of the rule we are very close to the start of the solving chain. We need one more Rule #4:
Now when we exclude the 4 in the bottom left box and square we final hit the solving chain. We only need Rule #1 to finish the puzzle.
This ends lesson 2 next is lesson 3.