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I am very confused about how the MIT license works in detail. In layman's terms, I understand the following:

  • If you distribute copies of MIT source, then you must include the license, and the rights in it are granted to the receiver.
  • If you modify MIT source (which is permitted) then you can distribute the modifications[1], but the derived work is not automatically MIT.

The MIT license only speaks in terms of copies.

  1. Must modified copies include the MIT license? Note that the modified copy might not even contain any code in common with the original - for example, if it was translated into another language.
  2. Is there any distinction when the code has not been modified per se, but is merely included as part of a larger work (e.g. suppose you copied an MIT file and pasted it inside a closed-source project)? I believe that the combined work is, technically, a derived work of both parts. If the answer to the first question is "no", then what about this case? (if it's "yes", then there is surely no distinction in this case)
  3. If so, does the MIT license thus included actually convey any rights? I think it must do, or I can't understand how the license worked for the original work. But I also think it must not, because if it did then my translated MIT library would appear to automatically be MIT (which is wrong).

Assume that the modified work is intended to be distributed as something other than MIT - for concreteness, let's say it's fully proprietary.

[1]: Incidentally, I don't see how this is implied exactly by the license text, but it seems everyone (including me) accepts it's allowed. The license refers only to "copies", so it looks like what it says is that you can modify copies and distribute copies, but not that you can distribute modified copies.

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Here is the text of the MIT license, quoted from OSI. I have changed angle brackets to square brackets so that it renders correctly:

Copyright [YEAR] [COPYRIGHT HOLDER]

Permission is hereby granted, free of charge, to any person obtaining a copy of this software and associated documentation files (the "Software"), to deal in the Software without restriction, including without limitation the rights to use, copy, modify, merge, publish, distribute, sublicense, and/or sell copies of the Software, and to permit persons to whom the Software is furnished to do so, subject to the following conditions:

The above copyright notice and this permission notice shall be included in all copies or substantial portions of the Software.

THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE.

Now to your questions:

Must modified copies include the MIT license? Note that the modified copy might not even contain any code in common with the original - for example, if it was translated into another language.

It costs you nothing to include the license, other than a tiny amount of UI that you can bury in a settings menu. There is no reason to avoid doing this, and there might be legal risk associated with it. If you really want to omit the license, I can only advise you to consult an attorney.

But I also think it must not, because if it did then my translated MIT library would appear to automatically be MIT (which is wrong).

The MIT license is not a copyleft license. It never requires you to give any sort of license to the recipient, and explicitly permits you to give them a different license if you so choose (that's what "sublicense" means).

In return for this rather generous allowance, the MIT license merely asks that you give the recipient a copy of its text. This is a purely informational requirement so that the recipient knows where the software originally came from; it does not cause your downstream work to also be licensed under MIT.

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  • If it's useful to you: You can escape the angle brackets with \< and \>. Jun 26 at 20:38
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    Can you clarify how the inclusion of the license text in the original code conveyed rights to the receiver, while the same license text included in the modified version is "purely informational"? Jun 26 at 20:44
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    @preferred_anon: Because the author of the original code intended for it to convey those rights, and placed it in a LICENSE file or some other place that is conventionally used for communicating that intent. If you put it under a label saying something like "License of [project name]" it's clear that you did not intend to provide such a license.
    – Kevin
    Jun 26 at 21:53
  • Sorry, I can't see what intent has to do with it. Imagine you include an MIT-licensed file inside a GPL project, for example. The MIT file has the original copyright notice in it, and the project has a LICENSE file including the GPL. According to your interpretation, I have to find out whether the GPL author's intent was to release the MIT file under MIT or GPL. I can't just read the file, because even though it includes the MIT license text, I have to know what the second author intends. Is that really true? Jun 28 at 21:23
  • @preferred_anon: If intent is obvious, then you can reasonably infer it. If not, you're rolling the dice on what a judge will decide.
    – Kevin
    Jun 28 at 21:33

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