Public Review Issue #48

Definition of "Directional Run"

There are two instances of the term "directional run" in UAX #9: The Unicode Bidirectional Algorithm. The more important one is the second: "Shaping is logically applied after the bidirectional algorithm is used, and limited to characters within the same directional run."

But the term "directional run" is not defined in the document. The proposal is to add a definition of directional run as follows:

BD3a Directional Boundary: The start or end of a sequence of characters, or any point in the sequence between two adjacent characters with different embedding directions. Thus there is a directional boundary between any two adjacent characters where one has an even (LTR) embedding level and the other has an odd (RTL) embedding level. BD3b Directional Run: A sequence of characters that starts and ends at directional boundaries, and otherwise contains no directional boundaries.