In March 2000 Richard Stallman (with Eben Moglen, a professor at Columbia Law School) introduced the GNU Free Documentation License (GFDL). Stallman identifies the GFDL as a means to enlist commercial publishers to fund free document writing without surrendering any vital liberty.
The GFDL identifies the conditions relating to the copying and revision of documents. Documents are more complicated than simply placing source code on the Internet. The GFDL is consequently more complex, covering the mass copying of the document, inclusion in collections, and specific issues relating to the covers.
Two important concepts relating to the availability of the document are introduced: transparent copies of the document and opaque copies of the document. A GFDL document must be transparent--that is, available in a format whose specification is available to the general public and which can be read using free software. Formats such as LATEX (used for this book) and XML (using publicly available DTDs) are fine. But making your document available only in PostScript or PDF or Microsoft Word is not transparent. These are opaque documents that might suffer the same old problems of document rot--after a few years the documents may no longer be accessible because the proprietor of the proprietary format might have gone out of business and the knowledge of the format has been lost.