Finally, the preamble introduces two terms used throughout the license to

clarify between the different types of derivative works: ``works that use

the library,'' and ``works based on the library.''  Unlike the GPL, the LGPL must

draw some lines regarding derivative works. We do this here in this

license because we specifically seek to liberalize the rights afforded to

copyright law would otherwise prohibit if the copyright holder exercised

his full permitted controls over the work.

\section{An Application: A Work that Uses the Library}

In the effort to allow certain proprietary derivative works and prohibit

``works based on the library,'' and ``works that use the library.''  The

distinction is drawn on the bright line of binary (or runtime) derivative

works and source code derivatives. We will first consider the definition

of a ``work that uses the library,'' which is set forth in LGPLv2.1~\S5.

We noted in our discussion of GPLv2~\S3 (discussed in

  Specifically, when she creates a running binary program, that running

  binary must be a derivative work, called \lplusi{}, that the user can

  run.

\item Since \lplusi{} is a derivative work of both \worki{} and \workl{},

  the license of \workl{} (the LGPL) can put restrictions on the license

\end{itemize}

We will talk about the specific restrictions LGPLv2.1 places on ``works

that use the library'' in detail in Section~\ref{lgpl-section-6}. For

now, focus on the logic related to how the LGPLv2.1 places requirements on

\section{The Library, and Works Based On It}

In short, a ``work based on the library'' could be defined as any

derivative work of LGPL'd software that cannot otherwise fit the

definition of a ``work that uses the library.''  A ``work based on the

library'' extends the full width and depth of copyright derivative works,

Most typically, one creates a ``work based on the library'' by directly

modifying the source of the library. Such a work could also be created by

tightly integrating new software with the library. The lines are no doubt

fuzzy, just as they are with GPL'd works, since copyright law gives us no

litmus test for derivative works of a software program.