&gt; <a href="https:&#x2F;&#x2F;www.gnu.org&#x2F;licenses&#x2F;rms-why-gplv3.html" rel="nofollow">https:&#x2F;&#x2F;www.gnu.org&#x2F;licenses&#x2F;rms-why-gplv3.html</a> seems to imply that that wouldn&#x27;t work, but I don&#x27;t understand why. Can someone with more experience with the GPLs enlighten me?<p>It doesn&#x27;t work to combine GPL code that way because both GPLv2 and GPLv3 would require the clause thus meaning a new &quot;GPLv2&quot; with that clause that isn&#x27;t technically GPLv2. Heck I don&#x27;t know their verbiage enough but they might not even be able to call it that.<p>However CDDL is not a strong copyleft, but a weak one, so would not require any update for the combination to work.<p>The real problem is that you would need a new hypothetical GPLv4 (since you wouldn&#x27;t want to call it GPLv3 even if you could) that includes some verbiage that allows this and then get everything that is GPLv3 that you include to upgrade to this new version. That is a pretty tall order to allow a slightly different copyleft license to exist.<p>In theory a new GPL version that allows coexistence of licenses could exist but doing that without allowing perverse &quot;copyleft&quot; licenses or specifically calling out some set of copyleft licenses would be really difficult.