diff --git a/.document b/.document
index fe7bc0def1..d02fec6fbc 100644
--- a/.document
+++ b/.document
@@ -1,4 +1,5 @@
 *.md
 *.rdoc
 lib
-doc
+doc/rdoc/*
+
diff --git a/doc/rdoc/rdoc_sources.md b/doc/rdoc/rdoc_sources.md
new file mode 100644
index 0000000000..798dde517c
--- /dev/null
+++ b/doc/rdoc/rdoc_sources.md
@@ -0,0 +1,77 @@
+# \RDoc Sources
+
+\RDoc derives Ruby documentation from files
+that are selected by \RDoc defaults and by user specifications.
+
+\RDoc reads files and directories specified as arguments on the `rdoc` command line;
+if no arguments are given, reads files and directories starting in `'.'`
+the current working directory.
+
+When anq argument is:
+
+- A file path: \RDoc reads the file.
+- A directory path:
+  \RDoc (by default) recursively reads all files and directories in the directory tree.
+  You can restrict the files and directories to be read;
+  see [Document-Control Files][document-control files].
+
+## File Types
+
+Based on the file extension of a file name,
+\RDoc assumes its content to be one of these:
+
+- `.rb`, `.rbw`: Ruby code.
+- `.c`, `.h`, `.C`, `.H`: C code.
+- `.rdoc`: Text file in `rdoc` (markup) format.
+- `.md`: Text file in `markdown` format.
+- `.rd`: Text file in `rd` format.
+- _Anything else_: Text file in `rdoc` (markup) format.
+
+In Ruby or C code, some documentation is derived from the code itself.
+
+Additional documentation may be derived from the text in comments;
+that text may be in any of these formats
+(see [directive :markup:][directive markup]).
+  
+- `rdoc` (markup);
+  see RDoc::MarkupReference.
+- `markdown`;
+  see RDoc::Markdown.
+- `rd`;
+  see [RD Working Draft][rd working draft].
+- `tomdoc`;
+  see [Tomdoc for Ruby][tomdoc for ruby].
+
+## Document-Control Files
+
+A directory may have a document-control file, named `.document`.
+
+- If a directory _does not_ have document-control file,
+  \RDoc reads all its files and subdirectories.
+- If directory _does have_ a document-control file,
+  \RDoc reads only the files and subdirectories specified there.
+
+The format of the document-control file is simple:
+
+- Each line may contain a _pattern_, which may be:
+
+    - A file path; e.g., `foo.rb`, `lib/bar.rb`.
+    - A directory path; e.g., `subdir`, `subdir/subsubdir`.
+    - A file glob; e.g., `*.rb`, `C*`, `lib/*.rb`.
+
+- A line may contain only one pattern (no whitespace- or comma-separated multiple patterns).
+- A pattern may be followed (on the same line) by a comment (beginning with `'#'`).
+- An entire line may be a comment (beginning with `'#'`).
+- A line may be blank (empty).
+
+Each directory in the filetree may (or may not) contain a document-control file.
+
+Note that a document-control file in a subdirectory is read _after_ document-control files
+in directories above it;
+therefore if file `parent/.document` specifies `child/foo.rb`,
+then `child/foo.rb` is read regardless of the content of `child/.document`.
+
+[document-control files]: rdoc-ref:rdoc_sources.md@Document-Control+Files
+[directive markup]: https://ruby.github.io/rdoc/RDoc/MarkupReference.html#class-RDoc::MarkupReference-label-3Amarkup-3A
+[rd working draft]: https://github.com/uwabami/rdtool/blob/master/doc/rd-draft.rd
+[tomdoc for ruby]: http://tomdoc.org