OpenBSD, the open source Unix-like operating system that today mostly lives in Linux’s shadow, turns 20 this month. To mark the occasion, here’s some historical background on one of the only major “open source” operating systems to have survived without embracing the GNU GPL license.

OpenBSD was born in October 1995. In important ways, however, its history stretches back much earlier than that. OpenBSD’s roots lie in the Berkeley Software Distribution, or BSD. BSD emerged at the University of California, Berkeley in 1978, when Bill Joy, who was a graduate student at the time, began developing the platform as an enhanced version of Unix. Unix itself had been under development at AT&T’s Bell Labs since 1969.

At first, Joy’s BSD project was a far cry from a standalone clone of Unix. The platform assumed greater import in the early 1980s, however, when AT&T decided to commercialize Unix. Disenchanted by that decision, the hacker community began working to implement a version of BSD that would be entirely free of Unix code and able to power a complete system on its own.

That effort was complete by 1991, when BSD developers released the Net/2 operating system. Net/2 BSD itself wasn’t designed for personal computers, but ports of the OS to Intel 386-based machines soon appeared. Plagued by legal troubles involving alleged misuse of code written for AT&T Unix, however, they failed to realize their full promise—which, incidentally, is a big part of the reason why Linux was able to grow into the most popular open source Unix-like OS, even though that was the last thing on Linus Torvalds’s mind when he began writing the Linux kernel in 1991.

Although the legal issues were not fully resolved until early 1994, BSD enthusiasts launched additional 386-based ports of the operating system beginning in the spring of 1993. One of them, NetBSD, made a name for itself by emphasizing compatibility with software related to the rapidly growing Internet. Another, FreeBSD, survives today as probably the most popular BSD-based OS.

Not everyone loved NetBSD, however. In 1995 Theo de Raadt, one of the leading NetBSD developers, left the project to found his own BSD distribution, OpenBSD. He and his followers set themselves apart by taking a purist approach to licensing. They rejected the GNU GPL, the software license that governed Linux and plenty of other important free-software projects, as too restrictive. They preferred instead the licensing terms of the original BSD operating system, which allowed developers essentially to do whatever they wanted with source code—including deny other developers the right to do the same by opting not to release the source code of their work.

The result was a BSD-based operating system that was almost entirely licensed under extremely permissive terms. The OpenBSD developers even rewrote the GPL-licensed components of the platform so that all of the software in their distribution would be compatible with BSD-style licenses.

That wasn’t because they didn’t love open code. The OpenBSD developers also made a commitment to publish all of their source code online, a practice that was rare at the time. They just liked BSD licensing better than the GPL.

The OpenBSD team also took documentation very seriously. In an era when free-software programmers were proving much better at writing free code than completing the more boring task of writing documentation to go along with it, that mattered.

Twenty years on, OpenBSD survives as one of the most popular alternatives to the world’s favorite alternative OS, Linux. Exactly how many people use it remains a mystery, since the project doesn’t publish usage statistics. But a report from 2005, which was probably the last time anyone studied this issue, suggested that around one-third of all users of BSD-based platforms were using OpenBSD.

Just in time to mark its 20th birthday, the OpenBSD project released OpenBSD 5.8 this week, which makes this an especially exciting time for BSD fans everywhere—however many there are out there.