As in , the message exists as a series of parts, each part being a group of lines containing characters in the character set employed.
1991年6月の MIME 原案では、メッセージは一連の部分として 存在し、各部は使われる文字集合中の文字からなる行の集団であります。
Each part may or may not be encoded using this format; we concern ourselves in this document solely with those that are. Within the relevant parts of the message, ordinary text may have occurrences of the following sequence: an intro character, followed by a string of characters that represent a character mnemonic, as described below. Similarly, the intro character may doubled, indicating a single occurence of the respective symbol in decoded format.
This message format uses the headers "Content-Type:" and "Content-Charset:" from  and is signified with the new header "Mnemonic-Intro:" and the two optional headers "Orig-Content-Charset" and "Orig-Mnemonic-Intro:". Case is ignored in these headers. A description of these headers and their fields follows.
このメッセージ書式は MIME 原案から「Content-Type:」と 「Content-Charset:」の各頭を、それから2つの任意頭 「Orig-Content-Charset」と「Orig-Mnemonic-Intro:」各頭を使います。 各頭・領域の説明は次の通りです。
If the Content-Type header is present, it must have the first header field as "Text" according to this specification. The Content-Type header can also be omitted, as Text is the default content type. The mnemonic message format is intended to be read by the end user possibly without further intervention.
The charset is given as one of the coded character set names in  and is the encoding used for the transport. For general use on the Internet, only "ASCII" and "ICS" are allowed.
charset は将来 RFC 1345 になる原案の中の符号化文字集合名の一つで、 転送に使われる符号化方法です。 Internet では通常の使用では、 「ASCII」と「ICS」だけが認められています。
ASCII is the recommended character set, while ICS will be very robust for traversing gateways, but it will cause trouble for (amongst other things) source code for several programming languages.
ASCII は推奨文字集合です。 ICS は関門を旅するのにとても 力強いでしょうが、プログラム言語のソース・コードなんかで 問題を起こすでしょう。
The use of other character sets are delimited to agreement between the communicating parties. When such an agreement has been achieved, or when a User Agent is operating in another character set than this transport character set, conversion of the message body part is done according to the tables in , as characters occuring in both encodings are just transformed, and characters not existing in the receiving coded character set are represented by the intro character of the receiving coded character set plus the mnemonic from , as described under the intro character.
他の文字集合の使用は通信団体間の合意によります。かような 合意のなされている時、あるいは利用者代理者が転送文字集合から他の文字集合 に変換しようとする時には、メッセージ本体部分の変換は 将来 RFC 1345 になる原案の表に沿い、両符号化方法で使える文字は 普通に変換し、受信側文字集合で存在しない文字は受信側符号化文字集合の 先導文字と将来 RFC 1345 になる原案の記法で表現します。
The characters forming the mnemonic are translated into the receiving code, which must have these characters present. An undefined character in the originating coded character set is transformed into a question mark character. The Content-Type:-header is changed accordingly to reflect such conversion. Conversion is not allowed if the content-type is not "Text" (which is the default if the Content-Type header is missing).
記法の文字形は受信符号に変換されますが、必ずそれらの文字表現を 持たなければなりません。元の符号化文字集合の未定義文字は 疑問符文字に変換されます。 Content-Type:(内容型) 頭は 変換を反映して修正されます。 content-type が「Text」(文字列) (Content-Type 頭が無い時の既定値) でない時は 変換は認められません。
An example of changing headers is the following: The UA runs in an 8-bit character set:
Content-Type: Text Content-Charset: ISO_8859-1 Mnemonic-Intro: 29 Orig-Content-Charset: ISO_8859-1 Orig-Mnemonic-Intro: 29
The MTA converts it before sending it to the recepient:
Content-Type: Text Content-Charset: ASCII Mnemonic-Intro: 38 Orig-Content-Charset: ISO_8859-1 Orig-Mnemonic-Intro: 29
The intro character is given as the decimal value of the intro character in the transport character set. The recommended value is 38 for the ampersand (&) character in ASCII. Another common value is 29 for the control character Group Seperator, which may be convenient when operat- ing in some environments, and ordinary text is not changed. The intro character is used for introducing character mnemonics from  when a character is not present in the mail transport character set (as defined by "charset"). Character mnemonics longer than two characters are sur- rounded by the underline character. The intro character is doubled to repesent one occurance of itself. Characters in the mail transport character set are normally just represented with their encoding, but may also be represented by the intro character and the mnemonic encoding. If the intro character is specified as 0 (zero), it is omitted in the transport, giving a better readably content, but eliminating the possi- bility of reversability and introducing an information loss.
The orig-charset is given as the original character set name. This may be set by the sending User Agent before converting the message into a character set suitable for transport. If no orig-charset is specified, the charset character set is used.
3.5. Orig-Mnemonic-Intro: orig-intro
The orig-intro character is given as the original intro character as used by the originating User Agent. The orig-charset and orig-intro may be used to recreate the message in its original encoding. If no orig- intro character is specified, the intro character is used. If the orig-intro character is specified as 0 (zero), no intro character was used in the original content. On the other hand, having a non-zero orig-intro value allows the user to generate characters, that are not available in the orig-charset.