Developing OpenType Fonts
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Appendix A: Writing System Tags
Features are encoded according to both a designated script and language system.
Currently most shaping engine implementations only support the "default" language system for each script. However, font developers may want to build language specific features which are supported in other applications and will be supported in future Microsoft OpenType implementations.
NOTE: It is strongly recommended to include the "dflt" language tag in all OpenType fonts because it defines the basic script handling for a font. The "dflt" language system is used as the default if no other language specific features are defined, or if the application does not support that particular language. If the "dflt" tag is not present for the script being used, the font may not work in some applications.
The following table lists the registered tag names for script and language systems. Note for new Indic shaping implementation “gur2” is used (old-behavior implementations used “guru”).
Note: both the script and language tags are case sensitive (script tags should be lowercase, language tags are all caps) and must contain four characters (ie. you must add a space to the three character language tags).