reading order
The reading order is the order a document is meant to be read in, which is not always the order its marks were drawn in.
In more detail
A page records where the ink went, one instruction after another. That sequence is whatever the program that wrote the file happened to produce: a footer may be drawn before the body, a caption before the picture, the right-hand column before the left.
For somebody looking at the page it makes no difference — the eye sees the layout. For anything reading the file rather than looking at it, the drawing order is all there is, unless the document states the order it means.
What it does on a real document
On a page in two columns the difference is plain: read in the wrong order, the first line of the left column is followed by the first line of the right, and the text turns to nonsense. The same happens on an invoice whose totals were drawn before its lines.
The order lives in the structure tree: the branches are walked in the order they were written, and that is the order the page is read in. Running heads, folios and rules are marked as decoration so that nothing reads them out at all.
Where it comes from, and why
It came in with the tagged PDF, and it is the reason tagging exists at all: everything else a tag says can be guessed at from the layout, and the order cannot.
The accessibility standard makes it a requirement rather than a courtesy, and the European rules on what public bodies publish point at that standard.
See it on a finished document
A document that can be read aloud The rules behind this word, and the texts they are read from
Glossary
- standard
- A rule argued out in committee, published under a number anybody may buy and read, and identical for every firm claiming it. A claim to follow one can therefore be checked against the text. A way of working that merely spread because it worked is a habit of the trade: useful, widespread, and answerable to no text at all.
- CMYK
- The four inks a printing press lays down: cyan, magenta, yellow and black. A screen mixes light instead, so a document going to a press states its colours in inks, not in screen colours.
- tagged PDF
- A PDF that carries the structure of its own text alongside the drawing: this is a heading, this is a table, this cell belongs to that column. It is what lets a reading aid, or a program pulling data back out, make sense of the page.
- structure tree
- The list, held inside a tagged PDF, that says what each part of the page is and what it belongs to: this block is a section, this line its heading, this run of cells one row of that table. It is the shape of the document, kept apart from the ink.