Precise terms for directionality in a writing system

Question

The term directionality often refers to two distinct directions in a writing system.

Most commonly directionality is used to refer to the direction in which individual characters are written in a line. For example, the directionality of latin script is left-to-right, the directionality of arabic script is right-to-left, mongolian or asian script is top-to-bottom, etc. Let's call this more precisely line direction / orientation.

However another direction is required to define a writing system, namely in which direction the lines progress. This "direction of line progression" is perpendicular to the line direction, which in a 2D plane means it can have two possible values. For example, in latin script the lines progress from top to bottom. But you could imagine a script where the lines start at the bottom and then stack upwards. And indeed, there are scripts with vertical line direction that differ in the "direction of line progression". For example, most asian script progresses right-to-left, while mongolian script progresses left-to-right.

It's this "direction of line progression" that's usually conflated with "directionality" and which I try to untangle. Something along the lines of line ordering, line progression direction, line progression order maybe?

For reference, this question uses "directionality" to refer to both of these example:

Word for the direction of written words

Wikipedia use the syntax right-to-left, top-to-bottom to describe the two directions on Right-to-left scripts. However they nowhere mention any terms for each. Unfortunately they also propose to shorten it to the first direction only,

(commonly shortened to right to left or abbreviated RTL)

which completely disregards that the second can still have two possible values (2 perpendicular directions on a 2D plane).

Answer 1

I've looked into this before, and I don't believe there is a specific standard terminology to describe all features of text directionality.

There are several different qualities involved across known languages. Just considering those which are broadly rectilinear in layout, there is the direction in which characters progress, there is the direction in which lines progress, there is the question of whether characters progress in the same direction on each line (and if not, whether they are mirrored or rotated on alternate lines), and perhaps other qualities that I do not recall offhand. Then you have even more unusual scripts which spiral or proceed circumferentially.

I would suggest that in cases where there is a need to distinguish character directionality from line directionality, you would have to distinguish them exactly like that.

I also stumbled upon a working group that have attempted to wrestle with some of these issues for the purposes of computer encoding of text: https://wiki.tei-c.org/index.php?title=Main_Page