+Here's a simple @samp{multipart/alternative}:
+
+@example
+<#multipart type=alternative>
+This is a plain text part.
+<#part type=text/enriched>
+<center>This is a centered enriched part</center>
+<#/multipart>
+@end example
+
+After running this through @code{mml-generate-mime}, we get this:
+
+@example
+Content-Type: multipart/alternative; boundary="=-=-="
+
+
+--=-=-=
+
+
+This is a plain text part.
+
+--=-=-=
+Content-Type: text/enriched
+
+
+<center>This is a centered enriched part</center>
+
+--=-=-=--
+@end example
+
+
+@node MML Definition
+@section MML Definition
+
+The @acronym{MML} language is very simple. It looks a bit like an SGML
+application, but it's not.
+
+The main concept of @acronym{MML} is the @dfn{part}. Each part can be of a
+different type or use a different charset. The way to delineate a part
+is with a @samp{<#part ...>} tag. Multipart parts can be introduced
+with the @samp{<#multipart ...>} tag. Parts are ended by the
+@samp{<#/part>} or @samp{<#/multipart>} tags. Parts started with the
+@samp{<#part ...>} tags are also closed by the next open tag.
+
+There's also the @samp{<#external ...>} tag. These introduce
+@samp{external/message-body} parts.
+
+Each tag can contain zero or more parameters on the form
+@samp{parameter=value}. The values may be enclosed in quotation marks,
+but that's not necessary unless the value contains white space. So
+@samp{filename=/home/user/#hello$^yes} is perfectly valid.
+
+The following parameters have meaning in @acronym{MML}; parameters that have no
+meaning are ignored. The @acronym{MML} parameter names are the same as the
+@acronym{MIME} parameter names; the things in the parentheses say which
+header it will be used in.
+
+@table @samp
+@item type
+The @acronym{MIME} type of the part (@code{Content-Type}).
+
+@item filename
+Use the contents of the file in the body of the part
+(@code{Content-Disposition}).
+
+@item charset
+The contents of the body of the part are to be encoded in the character
+set specified (@code{Content-Type}). @xref{Charset Translation}.
+
+@item name
+Might be used to suggest a file name if the part is to be saved
+to a file (@code{Content-Type}).
+
+@item disposition
+Valid values are @samp{inline} and @samp{attachment}
+(@code{Content-Disposition}).
+
+@item encoding
+Valid values are @samp{7bit}, @samp{8bit}, @samp{quoted-printable} and
+@samp{base64} (@code{Content-Transfer-Encoding}). @xref{Charset
+Translation}.
+
+@item description
+A description of the part (@code{Content-Description}).
+
+@item creation-date
+RFC822 date when the part was created (@code{Content-Disposition}).
+
+@item modification-date
+RFC822 date when the part was modified (@code{Content-Disposition}).
+
+@item read-date
+RFC822 date when the part was read (@code{Content-Disposition}).
+
+@item recipients
+Who to encrypt/sign the part to. This field is used to override any
+auto-detection based on the To/CC headers.
+
+@item sender
+Identity used to sign the part. This field is used to override the
+default key used.
+
+@item size
+The size (in octets) of the part (@code{Content-Disposition}).
+
+@item sign
+What technology to sign this @acronym{MML} part with (@code{smime}, @code{pgp}
+or @code{pgpmime})
+
+@item encrypt
+What technology to encrypt this @acronym{MML} part with (@code{smime},
+@code{pgp} or @code{pgpmime})
+
+@end table
+
+Parameters for @samp{text/plain}:
+
+@table @samp
+@item format
+Formatting parameter for the text, valid values include @samp{fixed}
+(the default) and @samp{flowed}. Normally you do not specify this
+manually, since it requires the textual body to be formatted in a
+special way described in RFC 2646. @xref{Flowed text}.
+@end table
+
+Parameters for @samp{application/octet-stream}:
+
+@table @samp
+@item type
+Type of the part; informal---meant for human readers
+(@code{Content-Type}).
+@end table
+
+Parameters for @samp{message/external-body}:
+
+@table @samp
+@item access-type
+A word indicating the supported access mechanism by which the file may
+be obtained. Values include @samp{ftp}, @samp{anon-ftp}, @samp{tftp},
+@samp{localfile}, and @samp{mailserver}. (@code{Content-Type}.)
+
+@item expiration
+The RFC822 date after which the file may no longer be fetched.
+(@code{Content-Type}.)
+
+@item size
+The size (in octets) of the file. (@code{Content-Type}.)
+
+@item permission
+Valid values are @samp{read} and @samp{read-write}
+(@code{Content-Type}).
+
+@end table
+
+Parameters for @samp{sign=smime}:
+
+@table @samp
+
+@item keyfile
+File containing key and certificate for signer.
+
+@end table
+
+Parameters for @samp{encrypt=smime}:
+
+@table @samp
+
+@item certfile
+File containing certificate for recipient.
+
+@end table
+
+
+@node Advanced MML Example
+@section Advanced MML Example
+
+Here's a complex multipart message. It's a @samp{multipart/mixed} that
+contains many parts, one of which is a @samp{multipart/alternative}.
+
+@example
+<#multipart type=mixed>
+<#part type=image/jpeg filename=~/rms.jpg disposition=inline>
+<#multipart type=alternative>
+This is a plain text part.
+<#part type=text/enriched name=enriched.txt>
+<center>This is a centered enriched part</center>
+<#/multipart>
+This is a new plain text part.
+<#part disposition=attachment>
+This plain text part is an attachment.
+<#/multipart>
+@end example
+
+And this is the resulting @acronym{MIME} message:
+
+@example
+Content-Type: multipart/mixed; boundary="=-=-="
+
+
+--=-=-=
+
+
+
+--=-=-=
+Content-Type: image/jpeg;
+ filename="~/rms.jpg"
+Content-Disposition: inline;
+ filename="~/rms.jpg"
+Content-Transfer-Encoding: base64
+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+
+--=-=-=
+Content-Type: multipart/alternative; boundary="==-=-="
+
+
+--==-=-=
+
+
+This is a plain text part.
+
+--==-=-=
+Content-Type: text/enriched;
+ name="enriched.txt"
+
+
+<center>This is a centered enriched part</center>
+
+--==-=-=--
+
+--=-=-=
+
+This is a new plain text part.
+
+--=-=-=
+Content-Disposition: attachment
+
+
+This plain text part is an attachment.
+
+--=-=-=--
+@end example
+
+@node Encoding Customization
+@section Encoding Customization
+
+@table @code
+
+@item mm-body-charset-encoding-alist
+@vindex mm-body-charset-encoding-alist
+Mapping from @acronym{MIME} charset to encoding to use. This variable is
+usually used except, e.g., when other requirements force a specific
+encoding (digitally signed messages require 7bit encodings). The
+default is
+
+@lisp
+((iso-2022-jp . 7bit)
+ (iso-2022-jp-2 . 7bit)
+ (utf-16 . base64)
+ (utf-16be . base64)
+ (utf-16le . base64))
+@end lisp
+
+As an example, if you do not want to have ISO-8859-1 characters
+quoted-printable encoded, you may add @code{(iso-8859-1 . 8bit)} to
+this variable. You can override this setting on a per-message basis
+by using the @code{encoding} @acronym{MML} tag (@pxref{MML Definition}).
+
+@item mm-coding-system-priorities
+@vindex mm-coding-system-priorities
+Prioritize coding systems to use for outgoing messages. The default
+is @code{nil}, which means to use the defaults in Emacs. It is a list of
+coding system symbols (aliases of coding systems does not work, use
+@kbd{M-x describe-coding-system} to make sure you are not specifying
+an alias in this variable). For example, if you have configured Emacs
+to prefer UTF-8, but wish that outgoing messages should be sent in
+ISO-8859-1 if possible, you can set this variable to
+@code{(iso-latin-1)}. You can override this setting on a per-message
+basis by using the @code{charset} @acronym{MML} tag (@pxref{MML Definition}).
+
+@item mm-content-transfer-encoding-defaults
+@vindex mm-content-transfer-encoding-defaults
+Mapping from @acronym{MIME} types to encoding to use. This variable is usually
+used except, e.g., when other requirements force a safer encoding
+(digitally signed messages require 7bit encoding). Besides the normal
+@acronym{MIME} encodings, @code{qp-or-base64} may be used to indicate that for
+each case the most efficient of quoted-printable and base64 should be
+used. You can override this setting on a per-message basis by using
+the @code{encoding} @acronym{MML} tag (@pxref{MML Definition}).
+
+@item mm-use-ultra-safe-encoding
+@vindex mm-use-ultra-safe-encoding
+When this is non-@code{nil}, it means that textual parts are encoded as
+quoted-printable if they contain lines longer than 76 characters or
+starting with "From " in the body. Non-7bit encodings (8bit, binary)
+are generally disallowed. This reduce the probability that a non-8bit
+clean MTA or MDA changes the message. This should never be set
+directly, but bound by other functions when necessary (e.g., when
+encoding messages that are to be digitally signed).
+
+@end table
+
+@node Charset Translation
+@section Charset Translation
+@cindex charsets
+
+During translation from @acronym{MML} to @acronym{MIME}, for each
+@acronym{MIME} part which has been composed inside Emacs, an appropriate
+charset has to be chosen.
+
+@vindex mail-parse-charset
+If you are running a non-@sc{mule} Emacs, this process is simple: If the
+part contains any non-@acronym{ASCII} (8-bit) characters, the @acronym{MIME} charset
+given by @code{mail-parse-charset} (a symbol) is used. (Never set this
+variable directly, though. If you want to change the default charset,
+please consult the documentation of the package which you use to process
+@acronym{MIME} messages.
+@xref{Various Message Variables, , Various Message Variables, message,
+ Message Manual}, for example.)
+If there are only @acronym{ASCII} characters, the @acronym{MIME} charset US-ASCII is
+used, of course.
+
+@cindex MULE
+@cindex UTF-8
+@cindex Unicode
+@vindex mm-mime-mule-charset-alist
+Things are slightly more complicated when running Emacs with @sc{mule}
+support. In this case, a list of the @sc{mule} charsets used in the
+part is obtained, and the @sc{mule} charsets are translated to @acronym{MIME}
+charsets by consulting the variable @code{mm-mime-mule-charset-alist}.
+If this results in a single @acronym{MIME} charset, this is used to encode
+the part. But if the resulting list of @acronym{MIME} charsets contains more
+than one element, two things can happen: If it is possible to encode the
+part via UTF-8, this charset is used. (For this, Emacs must support
+the @code{utf-8} coding system, and the part must consist entirely of
+characters which have Unicode counterparts.) If UTF-8 is not available
+for some reason, the part is split into several ones, so that each one
+can be encoded with a single @acronym{MIME} charset. The part can only be
+split at line boundaries, though---if more than one @acronym{MIME} charset is
+required to encode a single line, it is not possible to encode the part.
+
+When running Emacs with @sc{mule} support, the preferences for which
+coding system to use is inherited from Emacs itself. This means that
+if Emacs is set up to prefer UTF-8, it will be used when encoding
+messages. You can modify this by altering the
+@code{mm-coding-system-priorities} variable though (@pxref{Encoding
+Customization}).
+
+The charset to be used can be overridden by setting the @code{charset}
+@acronym{MML} tag (@pxref{MML Definition}) when composing the message.
+
+The encoding of characters (quoted-printable, 8bit etc) is orthogonal
+to the discussion here, and is controlled by the variables
+@code{mm-body-charset-encoding-alist} and
+@code{mm-content-transfer-encoding-defaults} (@pxref{Encoding
+Customization}).
+
+@node Conversion
+@section Conversion
+
+@findex mime-to-mml
+A (multipart) @acronym{MIME} message can be converted to @acronym{MML}
+with the @code{mime-to-mml} function. It works on the message in the
+current buffer, and substitutes @acronym{MML} markup for @acronym{MIME}
+boundaries. Non-textual parts do not have their contents in the buffer,
+but instead have the contents in separate buffers that are referred to
+from the @acronym{MML} tags.
+
+@findex mml-to-mime
+An @acronym{MML} message can be converted back to @acronym{MIME} by the
+@code{mml-to-mime} function.
+
+These functions are in certain senses ``lossy''---you will not get back
+an identical message if you run @code{mime-to-mml} and then
+@code{mml-to-mime}. Not only will trivial things like the order of the
+headers differ, but the contents of the headers may also be different.
+For instance, the original message may use base64 encoding on text,
+while @code{mml-to-mime} may decide to use quoted-printable encoding, and
+so on.
+
+In essence, however, these two functions should be the inverse of each
+other. The resulting contents of the message should remain equivalent,
+if not identical.
+
+
+@node Flowed text
+@section Flowed text
+@cindex format=flowed
+
+The Emacs @acronym{MIME} library will respect the @code{use-hard-newlines}
+variable (@pxref{Hard and Soft Newlines, ,Hard and Soft Newlines,
+emacs, Emacs Manual}) when encoding a message, and the
+``format=flowed'' Content-Type parameter when decoding a message.
+
+On encoding text, regardless of @code{use-hard-newlines}, lines
+terminated by soft newline characters are filled together and wrapped
+after the column decided by @code{fill-flowed-encode-column}.
+Quotation marks (matching @samp{^>* ?}) are respected. The variable
+controls how the text will look in a client that does not support
+flowed text, the default is to wrap after 66 characters. If hard
+newline characters are not present in the buffer, no flow encoding
+occurs.
+
+On decoding flowed text, lines with soft newline characters are filled
+together and wrapped after the column decided by
+@code{fill-flowed-display-column}. The default is to wrap after
+@code{fill-column}.
+
+
+
+
+@node Interface Functions
+@chapter Interface Functions
+@cindex interface functions
+@cindex mail-parse
+
+The @code{mail-parse} library is an abstraction over the actual
+low-level libraries that are described in the next chapter.