License Classification Summary

foss technology

This blogpost provides a summary of how I think of the spectrum of licenses from copyneutral/permissive licenses to copyleft/viral licenses. Of course, it is more complex than this, but this gives a good idea for use in discussions where the exact specifics are less relevant.

Generally, the idea behind copyleft licenses is that some organisation using and distributing a project dependent on the copyleft licensed project must also license their own with similar requirements to redistribute source code.

In practise, this is a spectrum as the requirements for redistribution of source can vary from nonexistent (ultra-permissive), to things such as “if you use it to host a network service, you must distribute it’s source to any users of that service if requested” (ultra-viral).

I describe licenses as varying degrees from copyneutral to viral based more granularly on the following two primary factors:

This isn’t in any way a full description of the licenses, but for the purposes of reasoning about their strength-of-copyleft, it is a decent metric even if it misses some of the specifics (for instance conflicts between FOSS licenses due to certain restrictions and the efforts of FOSS communities to prevent it via both multiversioning of licenses like “GPLv2+”, or changing of the licenses themselves to allow for it).

For me personally, the “stack” of licenses I tend to see and use (depending on how copyleft I think a project should be), with a focus on compatibility from top (most permissive) to bottom (most viral), is as follows:

For some larger licensing lists, take a look at the Free Software Foundation list and the Choose A License list.