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The Box Model
Margin Collapse

As you have seen, padding is space added inside an element’s border, while margin is space added outside an element’s border. One additional difference is that top and bottom margins, also called vertical margins, collapse, while top and bottom padding does not.

Horizontal margins (left and right), like padding, are always displayed and added together. For example, if two divs with ids #div-one and #div-two, are next to each other, they will be as far apart as the sum of their adjacent margins.

#img-one { margin-right: 20px; } #img-two { margin-left: 20px; }

In this example, the space between the #img-one and #img-two borders is 40 pixels. The right margin of #img-one (20px) and the left margin of #img-two (20px) add to make a total margin of 40 pixels.

Unlike horizontal margins, vertical margins do not add. Instead, the larger of the two vertical margins sets the distance between adjacent elements.

#img-one { margin-bottom: 30px; } #img-two { margin-top: 20px; }

In this example, the vertical margin between the #img-one and #img-two elements is 30 pixels. Although the sum of the margins is 50 pixels, the margin collapses so the spacing is only dependent on the #img-one bottom margin.

It may be helpful to think of collapsing vertical margins as a short person trying to push a taller person. The tall person has longer arms and can easily push the short person, while the person with short arms cannot reach the person with long arms.

Instructions

Study the graphic display to the right. Elements A and B have 20 pixels of horizontal margin, the sum of each element’s margin. Elements A and C have 30 pixels of vertical margin — the top margin of element C.

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