and to be processed relatively quickly. They also provide considerable flexibility. Inasmuch, however, as Apple's font tools require the generation of "morx" tables via raw state table information, they can be difficult to produce and debug. The font designer is also responsible for making sure that "morx" subtables are ordered correctly for the desired effect. Since AAT operates entirely with glyphs and never with characters, all the layout information necessary for producing the proper display resides within the font itself. This allows fonts to be added for new scripts without requiring any specific support from the OS. Third parties can produce fonts for scripts not officially supported by Apple, and they will work with macOS. On the other hand, this also means that every font for a given script requires its own copy of the script's shaping information in its own "morx" tables. Other AAT tables (or AAT-specific extensions to standard TrueType tables) allow for context-sensitive kerning, justification, and ligature splitting. AAT also supports variation fonts, in which a font's shape can vary depending on a scaled value supplied by the user. Variation fonts are similar to Adobe's defunct Multiple master fonts, where the endpoints are defined and any medial value is valid. With this, the user can then drag sliders in the user interface to make glyphs taller or shorter, to make them fatter or thinner, to increase or decrease the size of the serifs, and the like, all independently of one another. Glyphs may even have their fundamental shapes radically altered. Before OpenType introduced Font Variation in September 2016, there is nothing like this in OpenType. Other AAT tables can also have point-size dependent effects; for example, at 12 points, the horizontal and vertical strokes can be of similar width, but at 300 points, the stroke width variation could be quite great. In practice, few AAT fonts use any features of the technology other than those available through the "morx" table. Zapfino, Hoefler Text, and Skia are fonts that ship with macOS that illustrate a variety of AAT's capabilities. AAT for Indic scripts For Indic scripts, the only features that are necessary are glyph reordering and substitution; AAT supports both of these. As noted above, OpenType fonts for Indic scripts require AAT tables to be added before they will function properly on macOS. However, this applies only to software dependent on the system support of OpenType. Programs that