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If a library licensed under MIT licence is copied and extended on, should new additions contain the copyright notice as well?

Considering that the additions may at some point overwhelm the initial library, could the initial MIT licence ever change to some other licence?

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New additions to an MIT-licensed library don't preclude the inclusion of any additional copyright holders. So yes, you should include the new copyright declarations.

The MIT license is included in any derivative works. So if you use the library in your application, and your application has its own license, you still have to include the MIT license (and any copyright additions) in your work.

It's hard to imagine this getting too onerous. The folks making contributions to the library and holding the copyrights to those contributions deserved to be acknowledged, and having so many contributors to a library that the copyrights might actually consume a whole page of text is a good problem, not a bad one.

Xamarin (the company behind Mono) solves this problem (and some others) by requiring contributors to assign their copyright to Xamarin.

  • So who holds the copyright to new contributions of the open source library? Are these contributors forced to place their personal names on public copyright notices? – Trodder Apr 8 '15 at 17:01
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    I would imagine that most contributors to MIT projects don't really care about copyright for their particular contributions. As I said, Xamarin solves this problem by requiring contributors to assign their copyright to Xamarin, and the same could be done with any MIT open-source project. – Robert Harvey Apr 8 '15 at 17:03
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The MIT license only requires that the copyright notice is kept. It does not require you to keep the same license. And this makes sense: The MIT license wants you to do with the code whatever you want, but if you build upon the source code, it will still contain parts that were written by the original author(s). These authors retain the copyright for the parts they wrote, although they licensed the code to you under the MIT license.

A practical example: We have the following file:

/* Copyright (c) 2015 amon
 *
 * Permission is hereby granted
 * ...
 */

 A;
 B;

You now extend the program with new functionality by adding a step C. You hold the copyright for these changes. It would usually look like this:

/* Copyright (c) 2015 Trodder
 * Copyright (c) 2015 amon
 *
 * Permission is hereby granted
 * ...
 */

 A;
 B;
 C;

Since the MIT license allows you to sublicense the code, you can add your own license, but must keep the original copyright notice and original permission notice. I'd do it like this:

/* Copyright (c) 2015 Trodder
 * Copyright (c) 2015 amon
 *
 * You may use this code under the terms of the Wonderful-Trodder-Free-License (WTF-L)
 * ...
 *
 * This code builds upon previous effort by amon,
 * and is used under the following license:
 *
 * Permission is hereby granted
 * ...
 */

 A;
 B;
 C;

Technically, you hold the copyright over your changes even when you do not write a “Copyright <year> <name>” line. However, this makes it confusing for readers of that code to understand who currently holds the copyright over what parts of the file.

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