OpenType is a cross-platform font file format that has become the dominant way fonts are packaged and shipped. It was developed jointly by Microsoft and Adobe, beginning around 1996, as an extension of Apple and Microsoft’s earlier TrueType format. Its key unifying move was to support two different kinds of glyph outline within a single container: the quadratic curves of TrueType and the cubic curves of Adobe’s PostScript Type 1 fonts. This let the two long-competing font worlds converge on one format usable on both Windows and macOS.
According to the OpenType specification published by Microsoft, the format is “a widely-supported font format with a rich set of capabilities for digital typography.” It is built on the same “sfnt” table-based container structure as the original TrueType format, so well-formed older TrueType fonts are also valid OpenType fonts. Glyph outline data may use the TrueType glyph format or Adobe’s Compact Font Format (CFF or the newer CFF2).
OpenType’s most influential addition is its set of OpenType Layout tables, which the specification says “provide the advanced typographic capabilities needed for high-quality typography as well as for international text.” These tables drive features such as ligatures, contextual alternates, small capitals, old-style figures, and the kerning and positioning adjustments handled by the GPOS (glyph positioning) table. Crucially, the layout tables also encode the script- and language-specific shaping rules needed to render complex writing systems like Arabic and the Indic scripts.
The format has continued to evolve. The current specification is OpenType 1.9.1, and it has been aligned with the international standard ISO/IEC 14496-22 (“Open Font Format”), giving OpenType a formal home in the MPEG-4 family of standards. A major later addition was font variations, or “variable fonts,” which let a single font file produce a continuous range of weights, widths, or other design axes from one set of master outlines.
Because OpenType is supported by every major operating system and is the basis for the WOFF web-font wrappers used on the Web, it sits at the foundation of nearly all modern text rendering. Its design — one container, two outline formats, and a rich layer of layout features — resolved decades of font-format fragmentation and gave digital typography a common, capable standard.