This diagram illustrates a frame-like layout with 'position: fixed'.
The illustration shows a four-panel frameset. The top panel ("header") covers the entire width of the frameset and takes up 15% of the vertical space. Below it on the left is the "sidebar", which is 10em wide; it's height is automatically calculated. To the right of the sidebar and below the header is the "main" area, whose dimensions are computed based on remaining available space. Finally, at the bottom, the "footer" area is 100px high and covers the entire width of the frameset.
Each area is labeled and dimensions that are not 'auto' are indicated.