Margin collapsing occurs in three basic cases:
- Adjacent siblings</dt>
The margins of adjacent siblings are collapsed (except when the later sibling needs to be cleared past floats). For example:
<p>The bottom margin of this paragraph is collapsed...</p> <p>...with the top margin of this paragraph.<p>
- Parent and first/last child</dt>
- If there is no border, padding, inline content, or clearance to separate the margin-top of a block with the margin-top of its first child block, or no border, padding, inline content, height, min-height, or max-height to separate the margin-bottom of a block with the margin-bottom of its last child, then those margins collapse. </dd>
- Empty blocks</dt>
If there is no border, padding, inline content, height, or min-height to separate a block's margin-top from its margin-bottom, then its top and bottom margins collapse.
More complex margin collapsing (of more than two margins) occurs when these cases are combined.
When negative margins are involved, the size of the collapsed margin is the sum of the largest positive margin and the smallest (most negative) negative margin.