Clarification on the GNU Free Documentation License (GFDL) Applicability

[Peter X]

## Clarification on the GNU Free Documentation License (GFDL) Applicability

### Background

Hello,

I have been exploring the LilyPond 2.24.4 documentation as I plan to use
LilyPond along with LaTeX to write some music education books for sale on
Amazon. However, I have come across some concerns regarding the GNU Free
Documentation License (GFDL) mentioned at the end of the LilyPond
documentation. Below are the navigation details and the specific content of
the license:

**Navigation:**
```
<< Back to Documentation Index
LilyPond — Notation Reference v2.24.4 (stable-branch).
...
C. GNU Free Documentation License
...
```

**GFDL Content (Excerpt):**
```
Version 1.3, 3 November 2008
...
The purpose of this License is to make a manual, textbook, or other
functional and useful document free in the sense of freedom: to assure
everyone the effective freedom to copy and redistribute it...
```

### My Concerns

1. **Scope of Source Code Disclosure**:
   - According to the GFDL, if I use LilyPond and LaTeX to write a book and
sell it on Amazon, am I required to disclose the entire LaTeX source code
of the book, or just the parts that include LilyPond code? For instance, if
the book contains dozens or even hundreds of musical excerpts, do I need to
disclose all the LaTeX code or just the musical notations?

2. **Applicability of GFDL**:
   - Does the GFDL apply only to the documentation of LilyPond, or does it
also cover the software itself? This distinction is crucial for my decision
to use LilyPond for writing and publishing books.

3. **Use of Fonts**:
   - If I use the default fonts provided by LilyPond 2.24.4 to generate PDF
files, does this imply that I need to comply with the GPL license as well?
Would this require me to disclose the source code?

4. **Feasibility of Commercialization**:
   - If I must disclose the source code while selling the books, others can
easily recompile the source to generate the PDF, eliminating the need to
purchase the book. In this case, does it still make sense to use LilyPond
for writing and selling books?

### Request for Clarification

I would appreciate it if members of the forum could help clarify these
concerns. Specifically, I would like to understand:

- What exactly does the GFDL apply to? Is it limited to LilyPond's
documentation, or does it also encompass the software?
- If the GFDL only applies to the documentation, am I still required to
disclose the source code when writing and selling books?
- Are there any successful cases of using LilyPond to write and sell books
on Amazon? If so, how did they handle the issue of source code disclosure?

Thank you all for your time and assistance!