All paragraphs have a 10-pixel margin on the top and bottom. This paragraph has a 10 pixel margin on either side, as well. I gave the paragraphs borders so you could see the effects of the margins.

This paragraph has a 30-pixel margin on either side, pushing the sides away from the edges of the window.

This paragraph has a negative 30-pixel margin on either side, pushing the sides right out of the window. It also causes a horizontal scrollbar to appear at the bottom of the window.