
This is an example of diagonal "3" and "3". 


Here, assume an outer edge of the upper "3" has "x". 


Then the other three edges around that "3" will have lines.
And "x" will be marked on the edges out of the Lshape corner of the number. Now there are two "x" around "3" and this is not possible. 

Therefore this assumption is incorrect and that edge must have a line. 
Because of symmetric property, the outer edges of those two "3s" (four edges) always have lines.
Rule of diagonal "3 and 3"  
