A margin is the space between the CSS box and any box nearby. It's invisible and it's the outermost property of the box. The default margin is usually 0. Collapsing margins are what vertical margins do in relation to other margins they bump up against. If box A has a surrounding margin property of 8 and its next-door neighbor box B has one of 4, then CSS displays them as, not 4 or 12 units apart, but 8 units. No adding is done, vertically. The rule is that the larger value is used, the smaller is unused.