Thus, under the terms of LGPL, you must refrain from license terms on works

LGPLv3 is not surprisingly easier to understand and examine from a compliance

lens, since the FSF was influenced in LGPLv3's drafting by questions and

comments on LGPLv2.1 over a period of years.  Admittedly, LGPLv2.1 is still

in wide use, and thus compliance with LGPLv2.1 remains a frequent topic you

may encounter.  The best advice there is careful study of

Chapter~\ref{LGPLv2}.

However, to repeat a key point here made within that chapter: Note though

that, since the LGPLv2.1 can be easily upgraded to GPLv2-or-later, in the

worst case you simply need to comply as if the software was licensed under

GPLv2.  The only reason you must consider the question of whether you have a

``work that uses the library'' or a ``work based on the library'' is when you

wish to take advantage of the ``weak copyleft'' effect of the Lesser GPL\@.

If GPLv2-or-later is an acceptable license (i.e., if you plan to copyleft the

entire work anyway), you may find this an easier option.

You can also perform a similar same analysis through careful consideration of

the license text itself.  LGPLv2~\S2(a) states that if a licensed work is a

software library (defined in LGPLv2~\S0 as ``a collection of software

functions and/or data prepared so as to be conveniently linked with

application programs (which use some of those functions and data) to form

executables''), you have permission to distribute modified versions only if

those versions are themselves libraries.  LGPLv2.1 code can therefore not be

compliantly taken from its context in a library and placed in a non-library

modified version or work based on the work.  For its part, LGPLv2~\S6 does

not provide an exception for this rule: a combination may be made of a

modified version of an LGPL’d library with other code, but the LGPL’d code

must continue to be structured as a library, and to that library the terms of

the license continue to apply.

Either way you view the rules, these issues are admittedly complicated.

Nevertheless, In our years of work with the LGPLv2.1, however, we have never

seen a work of software that was not clearly one or the other; the line is

quite bright. At times, though, we have seen cases where a particularly large

work in some ways seemed to be both to both a work that used the library and

a work based on the library. We overcame this problem by dividing the work

into smaller subunits.  It was soon discovered that what we actually had were

three distinct components: the original LGPL'd work, a specific set of works

that used that library, and a specific set of works that were based on the

library. Once such distinctions are established, the licensing for each

component can be considered independently and the LGPLv2.1 applied to each

work as prescribed.

Finally, note though that, since the LGPLv2.1 can be easily upgraded to

GPLv2-or-later, in the worst case you simply need to comply as if the

software was licensed under GPLv2.  The only reason you must consider the

question of whether you have a ``work that uses the library'' or a ``work

based on the library'' is when you wish to take advantage of the ``weak

copyleft'' effect of the Lesser GPL\@.  If GPLv2-or-later is an acceptable

license (i.e., if you plan to copyleft the entire work anyway), you may find

this an easier option.