12.3.1 Display Areas
Display areas are areas that are not directly parts of lines. A display area has an inherent absolute orientation.


Informally, the box has an arrow on it saying this way up.
The positioning of display areas is specified by area containers.  An area container has its own coordinate system with its origin at the lower left corner, the positive x-axis extending horizontally to the right and the positive y-axis extending vertically upward.
An area container has a filling-direction specified in terms of its own coordinate system. The filling-direction gives a starting edge and an ending edge which are opposite to each other. The size of an area container is always fixed in the direction perpendicular to the filling-direction.  This means that the lengths of the starting and ending edges are always fixed and equal to each other.
The size of an area container in the filling-direction may be fixed or it may be specified to grow as necessary to contain the areas with which it is filled.  The display areas with which an area container is filled are always created so that their size in the direction perpendicular to the filling-direction is equal to the size of the area container in that direction.  This is called the display-size of the area.  An area container is filled with a sequence of display areas as follows. The first display area is positioned with its starting edge aligned with the area container's starting edge.  The next display area is then positioned with its starting edge on the previous area's ending edge, and so on.  This is illustrated in Figure 5, Area Containers and Display Areas.
An area container resulting from an included-container-area flow object may also specify a rotation to be applied to each of the display areas with which it is to be filled. The angle of rotation is restricted to be a multiple of 90 degrees. This rotation is applied to each display area, thus changing the display area's starting and ending edges.


It is possible to have paragraphs with lines with different  placement directions on the same page without using rotation.  See Figure 15, Multiple Filling Directions on a Single Page.
The direction between a display area's starting and ending edges is the placement direction of the display area. A display area also has an associated writing-mode that is perpendicular to the area's placement direction. This is illustrated in Figure 6, Placement Direction for Left-to-Right Writing-Mode.
Writing-mode may be left-to-right, right-to-left, or top-to-bottom.  See Figure 7, Different Writing-modes.