OpenType Design-Variation Axis Tag Registry (OpenType 1.9)
This registry defines design-variation axis tags for use in OpenType fonts. By providing registered tags with well-defined semantics and associated numeric scales for variation, this provides some degree of interoperability between different fonts from different vendors, or between fonts and applications.
Design-variation axis tags are used within the 'fvar' table in variable fonts, and also within the STAT table. Note that they are relevant for non-variable fonts as well as variable fonts: even though a font might not be a variable font, it is still a design variant within its font family. The STAT table allows applications to understand relationships of font design variants within a given family, whether they are implemented as non-variable fonts or as variation instances of a variable font.
Syntactic requirements for design-variation axis tags
Design-variation axis tags are four-byte arrays that can equivalently be interpreted as a string of four ASCII characters. (See Data Types for a general description of Tag values.) Axis tags must begin with a letter (0x41 to 0x5A, 0x61 to 0x7A) and must use only letters, digits (0x30 to 0x39) or space (0x20). Space characters must only occur as trailing characters in tags that have fewer than four letters or digits.
Fonts may use tags defined in this registry, or may use foundry-defined tags. (Foundry-defined tags can also be referred to as “custom” or “private” tags.) Foundry-defined tags must begin with an uppercase letter (0x41 to 0x5A), and must use only uppercase letters or digits. Registered axis tags must not use that pattern, but can use any other valid pattern. This ensures that foundry-defined tags and registered tags are never conflicting.
Documentation of registered axis tags
For every axis tag defined in this registry, certain information is required or recommended:
- The tag specification must include a US English name for the axis that may be used as a display string in application user interfaces to refer to the axis, or as the basis of localized display strings.
- The tag specification must include a description of the intended meaning and design-variation behavior for the axis.
- The tag specification must include information regarding the numeric scale used for the axis. This must include a specification of the range of values that are valid for the axis. Depending on the nature of the axis, this may or may not be a bounded range. It must also provide information regarding the semantic interpretation for values, specifying either some objective measure or some convention by which values can be interpreted.
- Whenever appropriate, the tag specification should also indicate a numeric value that is recommended or required (by definition of the scale) for that axis in a “Regular” font.
Note: The “Regular” font in a font family often has a sub-family name that omits axis qualifiers. For example, one foundry may create an optical-size font suited to 12 point size with “Text” included in the name as an indicator of the intended optical size, but another foundry may create a similar font without any indicator for optical size in the name. The choice for a recommended “Regular” axis value should be made with this in mind.
Additional information may also be provided, such as suggestions for programmatic selection of axis values that may be useful in applications.
The specification for the semantic interpretation of numeric values is required as a means to provide some degree of interoperability between different fonts, between fonts and software implementations, and between OpenType Font Variations and other specifications, such as font-weight values in CSS.
Note that interoperability is assumed to be attained in varying degrees, depending on the nature of an axis and the scale that it uses. For example, the scale for the Weight axis provides a limited degree of interoperability. Two different fonts with a Weight axis value of 700 (or “Bold”) may not result in the same amount of darkness or “color” when applied to the same text; but in both cases, a user can expect these to be darker than the “Regular” or “Semibold” fonts from each respective font family, and application developers can produce results that will be predictable for users if they associate that axis value with a particular state of a user-interface control or with a <strong> markup tag.
In contrast to this, the scale for the Optical size axis is designed to provide a much stronger degree of interoperability. For instance, two different fonts with an optical-size value of 20 are assumed to be best suited to text set at 20 points, because the scale is designed that way. If this axis had been defined with a different numeric scale, then an application might not be able to assume that two fonts with the same optical-size value are equally-well suited for a given context.
Not all axes will be equally amenable to a precise or objective measure. For example, there is no objective scale for an amount of italicness. But an Italic axis can be defined with a range from 0.0 to 1.0, representing whatever the font developer considers to be a non-italic design and a fully-italic design, and that is sufficient for applications to associate those numeric variation values with off/on states of an Italic toggle in a user interface to provide a meaningful and familiar experience. It also provides a useful basis of comparison between different fonts, which may be important, for instance, in font-fallback implementations: if the requested font face had an Italic axis setting of 1 but a font-fallback font must be used when displaying text, the application is able to select an appropriate Italic axis setting in the fallback font.
If an axis is intended to interact with programmatic mechanisms that automatically select axis values to provide some effect, then a more precise definition of the numeric scale and its interpretation may be needed. It must be clear to application and platform developers what independent variables should contribute as inputs for selection of axis values, and how the numeric values of the axis scale can be derived from those inputs.
For variable font implementations that support a given axis, the “Regular” value will often be a good choice for the default value of that axis in the 'fvar' variation axis record. The default values set in the 'fvar' table, however, are implementation-specific, and fonts are not required to use this “Regular” value as the axis default value.
Registered axis tags
The following design-variation axes and tags have been registered; the linked pages provide the axis tag descriptions. These are listed in alphabetic order of the tags.
| Axis tag | Name |
|---|---|
| 'ital' | Italic |
| 'opsz' | Optical size |
| 'slnt' | Slant |
| 'wdth' | Width |
| 'wght' | Weight |
How to register a design-variation axis tag
While font developers can always use foundry-defined axis tags however they might choose, Microsoft encourages font developers to use registered axis tags when implementing designs for which the registered axis is applicable. Microsoft welcomes nominations for new design-variation axis tag registration.
Registration of an axis tag can be useful for two key purposes. One is to foster conventionality and familiarity for a kind of design variation. For example, by defining the 'opsz' tag for optical-size variation, to use for tailoring glyph outlines according to the font size, different vendors can incorporate this kind of design variation into fonts, and the variations in these various fonts can be presented to font users as the same kind of variation. The more fonts that are implemented using an 'opsz' axis, the more familiar designers and content authors will become with this kind of design variation. They will benefit from greater consistency in experience when fonts use the same concepts than if different fonts were using similar but different concepts.
Another key purpose for registration of axis tags is to facilitate interoperability between different fonts or between fonts and applications. For example, by specifying a numeric scale for the 'opsz' axis that corresponds to text size in points, this makes it possible for applications to implement mechanisms for automatic selection of optical-size variation that can work with any fonts that support variation in the 'opsz' axis.
The merits for adding a variation axis tag to the registry is primarily determined in relation to these two key purposes: What is the likelihood that an axis of design variation will be implemented in fonts from multiple vendors and found to be useful to designers; and what is the likelihood that applications will implement mechanisms that make use of an interoperable understanding of the axis.
To qualify for registration, a complete description of the axis must be provided, including each of the categories of information listed above. If the axis is intended to interact with mechanisms that select axis values programmatically, then the description must include a clear specification for the numeric scale. There must be reasonable indication of alignment with one or both of the two key purposes for registration described above, and a reasonable indication that the axis will be implemented in fonts from multiple vendors and supported in software platforms and applications. It is recommended that the party proposing the new registration seek input from and get consensus among multiple font and software vendors regarding the definition of the proposed axis and its merits.