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                                           897
SECTION 10.7                                                                 Tagged PDF



Because the progression directions can vary depending on the writing system,
edges of areas and directions on the page must be identified by terms that are
neutral with respect to the progression order rather than by familiar terms such
as up, down, left, and right. Block layout proceeds from before to after, inline
from start to end. Thus, for example, in Western writing systems, the before and
after edges of a reference area are at the top and bottom, respectively, and the
start and end edges are at the left and right. Another term, shift direction (the
direction of shift for a superscript), refers to the direction opposite that for block
progression—that is, from after to before (in Western writing systems, from bot-
tom to top).

BLSEs are stacked within a reference area in block-progression order. In general,
the first BLSE is placed against the before edge of the reference area. Subsequent
BLSEs are stacked against preceding ones, progressing toward the after edge, until
no more BLSEs fit in the reference area. If the overflowing BLSE allows itself to be
split—such as a paragraph that can be split between lines of text—a portion of it
may be included in the current reference area and the remainder carried over to a
subsequent reference area (either elsewhere on the same page or on another page
of the document). Once the amount of content that fits in a reference area is de-
termined, the placements of the individual BLSEs may be adjusted to bias the
placement toward the before edge, the middle, or the after edge of the reference
area, or the spacing within or between BLSEs may be adjusted to fill the full ex-
tent of the reference area.

Note: BLSEs may be nested, with child BLSEs stacked within a parent BLSE in the
same manner as BLSEs within a reference area. Except in a few instances noted
below (the BlockAlign and InlineAlign elements), such nesting of BLSEs does not re-
sult in the nesting of reference areas; a single reference area prevails for all levels of
nested BLSEs.

Within a BLSE, child ILSEs are packed into lines. (Direct content items—those that
are immediate children of a BLSE rather than contained within a child ILSE—are
implicitly treated as ILSEs for packing purposes.) Each line is treated as a synthe-
sized BLSE and is stacked within the parent BLSE. Lines may be intermingled
with other BLSEs within the parent area. This line-building process is analogous
to the stacking of BLSEs within a reference area, except that it proceeds in the
inline-progression rather than the block-progression direction: a line is packed
with ILSEs beginning at the start edge of the containing BLSE and continuing
until the end edge is reached and the line is full. The overflowing ILSE may allow
itself to be broken at linguistically determined or explicitly marked break points

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