Directionality

If desired, horizontal text direction can be indicated using the following control sequences:

09/11 03/01 05/13	begin left-to-right text
09/11 03/02 05/13	begin right-to-left text
09/11 05/13	end of string

[This is a subset of the SDS (START DIRECTED STRING) control in the Draft Bidirectional Addendum to ISO 6429.]

Directionality can be nested. Logically, a stack of directions is maintained. Each of the first two control sequences pushes a new direction on the stack, and the third sequence (revert) pops a direction from the stack. The stack starts out empty at the beginning of a Compound Text string. When the stack is empty, the directionality of the text is unspecified.

Directionality applies to all subsequent text, whether in GL, GR, or an extended segment. If the desired directionality of GL, GR, or extended segments differs, then directionality control sequences must be inserted when switching between them.

Note that definition of GL and GR sets is independent of directionality; defining a new GL or GR set does not change the current directionality, and pushing or popping a directionality does not change the current GL and GR definitions.

Specification of directionality is entirely optional; text direction should be clear from context in most cases. However, it must be the case that either all characters in a Compound Text string have explicitly specified direction or that all characters have unspecified direction. That is, if directionality control sequences are used, the first such control sequence must precede the first graphic character in a Compound Text string, and graphic characters are not permitted whenever the directionality stack is empty.