RTF is Rich Text Format. The specification is published by Microsoft but it is publicly available for anyone to use so it is effectively "open".
We currently base our RTF off of the RTF Spec version 1.8, which is the version used in Word 2003 for Windows and Word 2004 for Mac. We also add a few features of our own for non-essential features. These extensions are added in the way defined by the RTF specification so our RTF files can be read by any standard RTF reader.
If you want the deep "geek" on the matter, check Microsoft's Rich Text Format (RTF) Specification, version 1.6, (248 printed pages) and enjoy.
What is "rtf", why do you use it and what version
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