Shows a paragraph with a top margin of 50px and a height of 150px;

Notice the height

Shows 2 paragraphs, each 50px with 50px top and bottom margin.

The bottom margin of this paragraph and the top margin of the bottom paragraph has collapsed into 1 margin of 50px

2*height + 1*margin-top + 1*margin-bottom) = x
100px + 50px + 50px = 200px