Blue Lines

How do i get to do those blue lines in the thread after someone reploes? Also, how do I delete

Like that blue line
Im not an expert here, but I noticed its usually when I make multiple replies before someone elsenreplies
Yes actualy it is
You can add them manually by typing the following in your post (in the unstyled/"View source" view if you're in the preview post page):


Like this:

That's what creates them.