Can I put a copyright notice on modified open source files?
<p>I have a software project which is essentially a heavily modified piece of open source software. I'm wanting to add copyright notices to the file headers to protect my work, as it is possible for users to extract the source code. Some files are totally original, some are modified, and some match the original open source project.</p>
<p>Can I put a &quot;Copyright &quot; notice on each file? If not, what about &quot;Subject to copyright - contact  for more information&quot; - would it be as effective of protection? If I put a &quot;Copyright &quot; notice on a file I didn't edit, what are the potential consequences?</p>

<p>Open source software is copyrighted by the contributors (or in some cases a legal entities the contributors explicitly gave their copyrights to in form of a <a href="https://en.wikipedia.org/wiki/Contributor_License_Agreement" rel="nofollow noreferrer">contributors license agreement</a>). Yes, they tend to give really lax licenses to everyone which grant a lot of freedoms, but the copyright is still retained.</p>
<p>When you modify open source software, then you create a combined work which contains both parts copyrighted by you and by the original creators. So you now have shared copyright. A good way to visualize that to downstream users is to add your copyright message to the copyrighted files <strong>in addition</strong> to the original copyright messages. But only on those files where you actually made changes.</p>

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