The SIL Open Font License (OFL) is an open source license designed for fonts and related software. SIL — a global non-profit dedicated to “sustainable language development... through research, translation, training and materials development” — created the OFL to encourage collaboration in font development and distribution.
The OFL allows for the use, modification, and redistribution of the licensed font and related software (collectively termed “font software”) — but with a few limitations.
For one, it prohibits the sale of licensed font software (or any individual components of the licensed font software) by itself. In other words, you can take an OFL-licensed font, use it in a software application, and then sell that software application. However, you could not sell a copy or modified version of OFL-licensed font software as a standalone product.
Additionally, derivative works (defined as “adding to, deleting, or substituting — in part or in whole — any of the components of the Original Version, by changing formats or by porting the Font Software to a new environment”) of OFL-licensed font software must also be released under the OFL.
The OFL includes a handful of other notable provisions, which we’ll cover later In this blog. We’ll also discuss the differences between OFL 1.0 and 1.1, and we’ll take a look at some of the more popular OFL-licensed fonts in use today.
SIL Open Font License 1.0 vs. 1.1
There are two versions of the SIL Open Font License: the original 1.0 (published in 2005), and the current 1.1 (published in 2007). SIL’s goal in publishing 1.1 was to “improve clarity and to fix some possible ambiguities.” Accordingly, most of the differences between the two versions are relatively minor.
For example, the preamble of OFL 1.0 includes the following sentence:
The fonts, including any derivative works, can be bundled, embedded, redistributed and sold with any software provided that the font names of derivative works are changed.
OFL 1.1 includes very similar phrasing, but with one change (emphasis added).
The fonts, including any derivative works, can be bundled, embedded, redistributed and/or sold with any software provided that the font names of derivative works are changed.
Here, SIL added “and/or” to make clear that each activity (bundling, embedding, redistributing) is permitted both as a standalone and/or in conjunction with selling.
Another minor difference involves the language used to describe “font software.” OFL 1.0 defines “font software” as referring to any and all “font files, data files, source code, build scripts, documentation.” OFL 1.1 updates the definition of “font software” to read “the set of files released by the Copyright Holder(s) under this license and clearly marked as such. This may include source files, build scripts and documentation.”
SIL explained that it made this change to add clarity and define “what is covered by what is marked with the OFL, not by the type of file.”
The most significant change (and the only actual operational change) between 1.0 and 1.1 covers font naming. Both versions of the OFL prohibit derivative works from using the “Reserved Font Name.” But the definition of “Reserved Font Name” changed from the OFL 1.0 to 1.1.
The relevant part of the original 1.0 reads as follows:
"Reserved Font Name" refers to the Font Software name as seen by users and any other names as specified after the copyright statement.
The updated part of 1.1 reads:
"Reserved Font Name" refers to any names specified as such after the copyright statement(s).
In addition to eliminating confusion over the phrase “seen by users,” the revised text requires authors to explicitly list any names they want to be “Reserved Font Names.” SIL notes that his change means “no font names are reserved by default.”
You can visit SIL’s website for a comprehensive list of differences between OFL 1.0 and 1.1.
SIL Open Font License Requirements
Like we mentioned, the SIL Open Font License (both versions) allows for the use, modification, and redistribution of the licensed font software, but only if certain conditions are met.
Derivative works of the licensed font software must include a copy of the license text and copyright notice. In situations where the font is embedded in a document (more on that below) or bundled in a program, it may be acceptable to replace the full license text with a link to the license text, but the SIL recommends against this. We suggest consulting the SIL’s website for more information on this requirement.
Derivative works may not use the name(s) of the original’s copyright holder(s) or author(s) to promote, advertise, or endorse without explicit permission.
Derivative works must also be released under the OFL. However, it is possible to bundle or aggregate OFL-licensed font software with software covered by other licenses. The SIL's license FAQ page notes that in this instance, “only the portions based on the Font Software are required to be released under the OFL.” 
Derivative works may not be sold in a standalone capacity. Per the SIL FAQ page, this restriction exists to “keep people from making money by simply redistributing the font.”
Derivative works must have a different “Reserved Font Name” than the original. As mentioned earlier in the blog, “Reserved Font Name” is defined in OFL 1.1 as “any names specified as such after the copyright statement.”
Other Notable SIL Open Font License Provisions
- The OFL is intended to be compatible with a broad range of open source licenses. OFL-licensed fonts can be distributed with copyleft (i.e. GPL v2, GPL v3, LGPL, AGPL, etc.) or permissive (Apache License 2.0, MIT License, etc) licensed software.
- The OFL does not consider embedding fonts in a document or file to meet the criteria for “distribution.” As such, the OFL’s requirements on font modification and redistribution don’t apply in this scenario. For context, SIL defines “embedding” on its FAQ page as follows:
“Inclusion of the font in a way that makes extraction (and redistribution) difficult or clearly discouraged. In many cases the names of embedded fonts might also not be obvious to those reading the document, the font data format might be altered, and only a subset of the font - only the glyphs required for the text - might be included.”
The Present and Future of the SIL Open Font License
The SIL Open Font License is both FSF (Free Software Foundation) and OSI (Open Source Initiative) approved. A large number of open source fonts are covered by the OFL, including:
- Gentium (and Gentium Basic and Gentium Plus)
- Charis SIL
- Source Sans Pro
Given the OFL’s popularity and provisions specifically designed for font software, there’s no reason why it won’t continue to be a favorite with font developers and users.
Note: We on the FOSSA Editorial Team are not lawyers. If you are seeking legal advice, we recommend you speak directly with an attorney.