I recently needed to create speech bubbles. To create the little tip at the end of the speech bubble, i used a css technique where the element is given a 0 width and 0 height and given borders. Making certain borders transparent would create diagonals.

This works great, and i am able to manipulate different borders to create triangles/shapes to my liking. The problem is, i don't fully understand why this works. I understand that it all starts out with a rectangle and as you set one or more of the borders to transparent it creates a diagonal. My question is how does this work ? And why does it create a diagonal in the first place ?

EDIT: To clarify, i would like to know the theory behind why this technique works.