Last in a list of the top 10 free/open source software legal developments in 2011 (emphasis added):

Open Hardware License. The open hardware movement received a boost when CERN published an Open Hardware License (“CERN OHL”). The CERN OHL is drafted as a documentation license which is careful to distinguish between documentation and software (which is not licensed under the CERN OHL) http://www.ohwr.org/documents/88. The license is “copyleft” and, thus, similar to GPLv2 because it requires that all modifications be made available under the terms of the CERN OHL. However, the license to patents, particularly important for hardware products, is ambiguous. This license is likely to the first of a number of open hardware licenses, but, hopefully, the open hardware movement will keep the number low and avoid “license proliferation” which has been such a problem for open source software.

But the CERN OHL isn’t the first “open hardware license”. Or perhaps it is the nth first. Several free software inspired licenses intended specifically for open hardware design and documentation have been created over the last decade or so. I recall encountering one dating back to the mid-1990s, but can’t find a reference now. Discussion of open hardware licenses was hot at the turn of the millennium, though most open hardware projects from that time didn’t get far, and I can’t find a license that made it to “1.0”.

People have been wanting to do for hardware what the GNU General Public License has done for software and trying to define open hardware since that timeframe. They keep on wanting (2006) and trying (2007, 2011 comments).

Probably the first arguably “high quality” license drafted specifically for open hardware is the TAPR Open Hardware License (2007). The CERN OHL might be the second such. There has never been consensus on the best license to use for open hardware. Perhaps this is why CERN saw fit to create yet another (incompatible copyleft at that — incompatible with TAPR OHL, GPL, and BY-SA), but there still isn’t consensus in 2012.

Licenses primarily used for software (usually [L]GPL, occasionally BSD, MIT, or Apache) have also been used for open hardware since at least the late 1990s — and much more so than any license created specifically for open hardware. CC-BY-SA has been used by Arduino since at least 2008 and Qi since 2009.

In 2009 the primary drafter of the TAPR OHL published a paper with a rationale for the license. By my reading of the paper, the case for a license specific to hardware seems pretty thin — hardware design and documentation files, and distribution of printed circuit boards seem a lot like program source and executables, and mostly subject to copyright. It also isn’t clear to me why the things TAPR OHL handles differently than most open source software licenses (disclaims strictly being a copyright license, instead wanting to serve as a clickwrap contract; attempts to describe requirements functionally, instead of legally, to avoid describing explicitly the legal regime underlying requirements; limited patent grant applies to “possessors” not just contributors) might not be interesting for software licenses, if they are interesting at all, nor why features generally rejected for open source software licenses shouldn’t also be rejected for open hardware (email notification to upstream licensors; a noncommercial-only option — thankfully deprecated late last year).

Richard Stallman’s 1999 note about free hardware seems more clear and compelling than the TAPR paper, but I wish I could read it again without knowing the author. Stallman wrote:

What this means is that anyone can legally draw the same circuit topology in a different-looking way, or write a different HDL definition which produces the same circuit. Thus, the strength of copyleft when applied to circuits is limited. However, copylefting HDL definitions and printed circuit layouts may do some good nonetheless.

In a thread from 2007 about yet another proposed open hardware license, three people who generally really know what they’re talking about each wondered why a hardware-specific license is needed: Brian Behlendorf, Chris DiBona, and Simon Phipps. The proposer withdrew and decided to use the MIT license (a popular non-copyleft license for software) for their project.

My bias, as with any project, would be to use a GPL-compatible license. But my bias may be inordinately strong, and I’m not starting a hardware project.

One could plausibly argue that there are still zero quality open hardware specific licenses, as the upstream notification requirement is arguably non-open, and the CERN OHL also contains an upstream notification requirement. Will history repeat?

Addendum: I just noticed the existence of an open hardware legal mailing list, probably a good venue to follow if you’re truly interested in these issues. The organizer is Bruce Perens, who is involved with TAPR and is convinced non-copyright mechanisms are absolutely necessary for open hardware. His attempt to bring rigor to the field and his decades of experience with free and open source software are to be much appreciated in any case.