This question stems from discussion of a question on programmers.stackexchange.com.In the LGPL 3.0 section 4 Combined Works it states:You may convey a Combined Work under terms of your choice that,  taken together, effectively do not restrict modification of the  portions of the Library contained in the Combined Work and reverse  engineering for debugging such modifications, if you also do each of  the following:Generally, it seems the LGPL license is concerned with open source libraries and software that uses these libraries where a combined work would be a piece of software that includes a library.If one were to open source a standalone single file program and someone modified that and redistributed it, it would technically be a derivative work. Would the statements regarding combined works map to derivative works?Or more simply: Are derivative works a subset of combined works?
Are derivative works a subset of combined works?
derivative works;lgpl
There are two separate kinds of derived works you can create from an LGPL-licensed work:a combined work that includes/uses the LGPL library (per section 4, Combined Works)a modification of the LGPL library itself (per section 2, Conveying Modified Versions)To qualify as a Combined Work, your changes cannot be based on the library, but rather must be a separable component that uses the library as an interface. Consider the definitions from the license (emphasis mine):An Application is any work that makes use of an interface provided by the Library, but which is not otherwise based on the Library. Defining a subclass of a class defined by the Library is deemed a mode of using an interface provided by the Library.A Combined Work is a work produced by combining or linking an Application with the Library.So, a combined work consists ofthe LGPL-licensed library, andanother work (Application) that is not based on the libraryVisually, we could represent the whole Combined Work as: [Application] === (uses) ===> [Library]|                                       ||-------------Combined Work-------------|It seems to me (note: not a lawyer) that if a derived work is only a modification of the library itself, it would qualify as being based on the library. Therefore, such a work could not be part of a union that satisfies the license's definition of a Combined Work, because its non-Library component is based on the text Library, rather then using an interface provided by the Library. Instead, it would be a modification of the library, and governed by section 2, rather than section 4.On the other hand, if you modified the library's file to add completely separate code that merely uses the library's interface, that wouldn't seem to be based on the library, because the two components (library and your addition) are completely separate (but merely happen to be contained inside the same file). For example:class OriginalLibrary() {    void someInterface(int foobar) {        return new Sys().doStuff(foobar, 18);    }}int main(char** argv, int argc) {    OriginalLibrary mylib = new OriginalLibrary();    printf(I use the library, but I'm not pat of it);    printf(mylib.someInterface(argv[0]));}In this case, the int main method is clearly not based on the OriginalLibrary class, even if you put them in the same file.Finally, your work could be both a modification and a combined work. You could first modify the library itself, and then include that modified library inside of a larger combined work.