DFSG (Debian Free Software Guidelines)
Debian's ten criteria for what counts as free software — and the template the Open Source Definition was copied from.
Bruce Perens drafted the guidelines, and Debian adopted them in 1997 as part of its Social Contract. Where the Free Software Foundation spoke in terms of freedoms, Perens wrote something you could test a licence against clause by clause: free redistribution, source code included, derived works allowed, no discrimination against persons or fields of endeavour. A year later the text had its Debian references scrubbed and became the OSI's definition of open source.
The project also produced rules of thumb now used well beyond it, such as the desert island test: could a castaway with no way of sending changes upstream still comply with the licence? In 2006 Debian ruled the GNU FDL with invariant sections non-free, which says something about how literally it reads its own rules. Whatever fails ends up in the non-free archive area.