An idea that is suggested every now and then is to look at software licensing and give it a kind of "Creative Commons" feel; that is, present the terms of the license in a pleasant and orderly way by means of icons. Now, we've already come to the realization that calling something "Creative Commons licensed" is vague to the point of being useless (just "some rights reserved"). Calling something "Free Software" is also vague, but there is a rock-solid guarantee at the bottom: the term guarantees you, the recipient of the software, at least the Four Freedoms. Any Open Source software you receive usually means at least the Four Freedoms as well. So you need to say which CC, which Free Software license, which Open Source license.
CC has six licenses; they are split neatly and orthogonally along the commercial / non-commercial and yes / share-alike / no axes.
The thing is, CC is a much simpler system because it applies to work where there are no patent concerns, where embedded systems don't have a place, where share-alike has a simpler meaning. I have trouble bringing this same simplicity to software licensing, but I thought I would give it a try.
First off, every Free Software license gives you the right to run (presumably also to compile and then run), study, and modify the work; it must be possible to (re)distribute the (modified) work. So those are the basic permissions. We could put them under a basic "Free Software" symbol, like I've done here.
Now we get to modifiers of the basic license. What is allowed by one Free Software license, and not allowed by another? Where do the licenses differ on essential points? Going through such an exercise opens up the debate on what constitutes an essential point of difference between licenses. Still, I think we can agree that many licenses ask you to deliver the complete corresponding source along with a binary. Details (written offers, etc.) differ greatly though. In the interest of simplicity, though, we'll just lump it all together as "publish the source". The BSD License allows binary distribution without publishing the source, so it doesn't get this symbol.
Now to distinguish strong copyleft from weak copyleft -- that's really important when you want to know what the effect is on your own code and own license choices if you are going to incorporate another piece of Free Software into yours. I suppose, actually, that we don't need to distinguish this dimension from the previous "publish the source" dimension: I think every "publish the source" license is also one of the two kinds of copyleft (although I can imagine a license that says you must re-publish the original source, but not necessarily your modifications).
So how about Tivoization? Or in other words, embedding into a device and then selling, renting or lending the device instead of delivering the software sec? It's a real difference between GPLv2 and GPLv3. It's explicitly mentioned in some faux Free Software licenses that allow use except when embedded. I call them faux (fake) because Freedom 0 is the Freedom to use, for any purpose, and clauses 5 and 6 of the Open Source Definition do much the same. So let's add that dimension into the mix.
I'll throw in patent grants as another factor. This is an over-broad blanket, because the subtleties of patent licensing are devilish. The Apache License, version 2 for instance contains a patent grant with a termination clause. So does the CDDL.
So, let's take a look (squinting through this rather imperfect telescope) at the big picture. We'll take the top 10 licenses (according to Black Duck Software) and for each, label it with icons like these and comment on what's been missed out by the icon scheme. Note that these are not necessarily all Free Software licenses, and not all of them are widely used across the Free Software ecosystem.
"The original and best" Most widely used license, apparently applied to nearly 50% of all Free Software projects. I imagine that 50% also comes from things like "v2 or (at your option) any later version" licensing.
Artistic is applied to a huge number of Perl modules, which are counted individually, which is why Artistic shows up as a significant license force, even if it is almost unused outside of the Perl community.
- BSD 2-clause
The Tivoization clause (part of section 6) can be disabled in the GPLv3 by granting additional permissions in accordance with section 7, if you really want.
- Apache 2.0
- Code Project Open 1.02
The CPOL is a strange license. It is not OSI approved, as near as I can tell. It seems to disallow the distribution of modified works, certain uses are disallowed by the license (immoral ones), there's a no-sale clause, indemnity, and some other bits that make this license difficult for me to place anywhere in the world of Free Software.
The MS-PL is kind of strange; I've never seen it in practice. It looks roughly -- very roughly -- like BSD plus a patent clause. This is a Free Software license, but GPL-incompatible.
Comments on my artistic icon skills should be addressed to Nuno Pinheiro. You may be able to hire him to do very nice icons for this set-up, and Björn Balazs can do usability testing on them. Kolourpaint FTW. Now, as for the accuracy of this table, I'll say it's a best-effort one-morning overview, so there may be plenty of errors in there. The point is the principle of reducing the licenses to a sequence of icons. The icon for patent is a patent troll (notice the glowing red eyes) because I couldn't think of anything better.
So, errors and omissions aside -- I welcome corrections in the comments on this blog -- we need to ask the question: does this scheme of badges highlight any (all?) of the essential differences between the different licenses? If not, what additional discriminatory characteristic should we add to distinguish them?
Based on this list, we see that BSD 2-clause and MIT are "the same". Are they really? Well, it depends on how you interpret the second clause of the BSD 2-clause license and whether the single MIT clause implies it. In a world of good faith, you could satisfy the MIT license by doing what the BSD 2-clause license asks you to do. So I think I could be satisfied that this is a same-difference identification of two licenses.
But we could look at some others -- is Apache equivalent to Mozilla in all meaningful ways? How about LGPLv2.1 and Artistic? That, however, will have to wait for another day. Where I set out to demonstrate that you can't reduce licenses to blurbs and icons, I haven't done so yet -- and still, such a reduction might be useful from a license selection standpoint, because you can pick and choose based on broad categories of license behavior.