Each language system tag consists of three uppercase letters, followed by a single space. This section defines the standard TrueType Open baseline tags that Microsoft supports. A registered baseline tag has a specific meaning and conveys information to font users about a baseline's use. For compatibility and ease of use, Microsoft encourages font developers to use registered baseline tags. This version of the Tag Registry identifies the baselines that Microsoft has implemented to date.
Baseline tags consist of four lowercase letters.
Features provide information about how to use the glyphs in a font to render a script or language. For example, an Arabic font might have a feature for substituting initial glyph forms, and a Kanji font might have a feature for positioning glyphs vertically. All TrueType Open features define data for glyph substitution, glyph positioning, or both. Each TrueType Open feature has a feature tag that identifies its typographic function and effects. By examining a feature's tag, a text-processing client can determine what a feature does and decide whether to implement it.