RTF Rich Text Format
RTF (Rich Text Format) is Microsoft's text-based interchange format, created in the late 1980s so formatted documents could move between applications and operating systems that shared nothing else. Its distinguishing property is that it is plain text.
An RTF file is ASCII, with formatting expressed as control words inside brace-delimited groups, each group naming the text it affects and the attributes applied to it. Non-ASCII characters are escaped into the appropriate code values, which is what keeps the file transportable.
The specification stopped moving: version 1.9.1 in March 2008 was the last, and Microsoft no longer extends it. That freeze is exactly why RTF remains a dependable lowest common denominator, and equally why it cannot express what a modern format can.[1][2][3]