The open-source, community-maintained game Simutrans uses the Artistic Licence 1.0 (AL). I know many people regard this as a poor choice, but the core of the game was written as someone's spare time project in the 1990s, so it's two decades too late for relicensing.* However, compiling it from source now requires several libraries that use the zlib Licence (zL). Simutrans has been careful not to distribute those libraries with its source code, because of the difference in licences. This is confusing for newcomers, especially since there are many possible combinations of game versions and library versions, which do not all play nicely together. However, licences can be different, but compatible. Are the AL and the zL compatible for this purpose?

There are several answers here that address various compatibility issues, but none deal with this particular combination. The "choosealicense" website has a blank space in the "same license" column for both these licences, suggesting that neither requires that it be distributed only with other code from the same licence.

Note that we are talking about the Simutrans source and the library binaries and source; it seems to be well established that a zL binary can be integrated into an AL binary. The zL also seems very permissive, and does not pose any obstacle. So it comes down to the notorious clause 4 of the AL. Yet it seems to me that clause 4(a) was written to cover precisely this situation:

  • the software is being modified in accordance with the wishes of the Copyright Holders, so what is being distributed is the Standard Version source code in the future;

  • all that is necessary is to make clear where people of the future can obtain the last source code revision that does not include the zlib libraries (the Standard Version of the present).

However, IANAL and would welcome an answer from someone with more expertise.

*N.B.: This also means that the clauses 3(d) and 4(d) of the AL cannot be used, because many of the Copyright Holders are uncontactable.

1 Answer 1


The Artistic License is at its heart an extremely liberal license with few restricitions, even more so than the MIT/BSD license family. But it also enforces trademark-like properties intended to give the original author artistic control over their work, which gives the license a confusing copyleft flavour. The solution is to look carefully at your use case.

If the community-maintained version is the Standard Version, you are in luck. Binaries of the project can be distributed including any libraries under clause 4(a):

You may distribute the programs of this Package in object code or executable form, provided that you […] distribute a Standard Version of the executables and library files, together with instructions (in the manual page or equivalent) on where to get the Standard Version.

Your interpretation therefore seems to be correct.

If it is not the Standard Version, you can still distribute binaries of the software including libraries, if you, at your choice:

  • 4(b): include the source code of your version in the download (this doesn't seem to extend to the source code of libraries), OR

  • 4(c): distribute your version under a different name, document differences to the Standard Version (e.g. in a changelog), and point to the Standard Version.

If you are concerned about distributing the source code of the project along with the source or executable form of libraries, then clause 4 is irrelevant. Clause 1 lets you make verbatim copies of the standard version, and clause 3 covers copies of the modified version. However, no restrictions apply to the development of the Standard Version itself.

In any of these cases, it is possible to use any library that does not impose a particular license for the combined software (like the GPL would do). But some care has to be taken. The licenses of the libraries may require certain notices to be retained or displayed. The AL does not generally require this. So you are not able to distribute the project + libraries under only the AL. Instead the complete license for such a distribution must therefore consist of both the AL and all library licenses, each of which only apply to their respective parts. Users must comply with all of these licenses at the same time.

Such a combined license document doesn't have to be complicated. You can point to the full license terms in a different document. E.g.:

Copyright 1997 Copyright Holder

SomeProject is licensed under the terms of the Artistic License, which can be found in doc/artistic.txt.

libs/some/ contains SomeLibrary:

Copyright 2003 SomeoneElse

Put the SomeLibrary notice here

libs/other/ contains SomeOtherLibrary:

Copyright 1995 SomeOtherAuthor

Put the SomeOtherLibrary notice here

Note that the Artistic License definition of a Copyright Holder can be distinct from the actual copyright holders. The AL Copyright Holder is whoever is named explictly in the copyright notice, which is often only the original author. Other contributors may hold copyright for their contributions, without having the rights of a Copyright Holder under the AL. To enter into alternative distribution agreements as per clause 4(d), you only need permission from the Copyright Holder, not from all copyright holders.

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