The Containing Block

Every element is laid out with respect to its containing block; in a very real way, the containing block is the "layout context" for an element.For an element in the normal, Western-style flow of text, the containing block is formed by the content edge of the nearest block-level, table cell, or inline-block ancestor box. You don't need to worry about inline elements since the way they are laid out doesn't depend directly on containing blocks.