7.1 The Text Content-Type

The text Content-Type is intended for sending material which is principally textual in form. It is the default Content- Type. A "charset" parameter may be used to indicate the character set of the body text. The primary subtype of text is "plain". This indicates plain (unformatted) text. The default Content-Type for Internet mail is "text/plain; charset=us-ascii".

Beyond plain text, there are many formats for representing what might be known as "extended text" -- text with embedded formatting and presentation information. An interesting characteristic of many such representations is that they are to some extent readable even without the software that interprets them. It is useful, then, to distinguish them, at the highest level, from such unreadable data as images, audio, or text represented in an unreadable form. In the absence of appropriate interpretation software, it is reasonable to show subtypes of text to the user, while it is not reasonable to do so with most nontextual data.

Such formatted textual data should be represented using subtypes of text. Plausible subtypes of text are typically given by the common name of the representation format, e.g., "text/richtext".

7.1.1 The charset parameter

A critical parameter that may be specified in the Content- Type field for text data is the character set. This is specified with a "charset" parameter, as in:

Content-type: text/plain; charset=us-ascii

Unlike some other parameter values, the values of the charset parameter are NOT case sensitive. The default character set, which must be assumed in the absence of a charset parameter, is US-ASCII.

An initial list of predefined character set names can be found at the end of this section. Additional character sets may be registered with IANA as described in Appendix F, although the standardization of their use requires the usual IAB review and approval. Note that if the specified character set includes 8-bit data, a Content-Transfer- Encoding header field and a corresponding encoding on the data are required in order to transmit the body via some mail transfer protocols, such as SMTP.

The default character set, US-ASCII, has been the subject of some confusion and ambiguity in the past. Not only were there some ambiguities in the definition, there have been wide variations in practice. In order to eliminate such ambiguity and variations in the future, it is strongly recommended that new user agents explicitly specify a character set via the Content-Type header field. "US-ASCII" does not indicate an arbitrary seven-bit character code, but specifies that the body uses character coding that uses the exact correspondence of codes to characters specified in ASCII. National use variations of ISO 646 [ISO-646] are NOT ASCII and their use in Internet mail is explicitly discouraged. The omission of the ISO 646 character set is deliberate in this regard. The character set name of "US- ASCII" explicitly refers to ANSI X3.4-1986 [US-ASCII] only. The character set name "ASCII" is reserved and must not be used for any purpose.

NOTE: RFC 821 explicitly specifies "ASCII", and references an earlier version of the American Standard. Insofar as one of the purposes of specifying a Content-Type and character set is to permit the receiver to unambiguously determine how the sender intended the coded message to be interpreted, assuming anything other than "strict ASCII" as the default would risk unintentional and incompatible changes to the semantics of messages now being transmitted. This also implies that messages containing characters coded according to national variations on ISO 646, or using code-switching procedures (e.g., those of ISO 2022), as well as 8-bit or multiple octet character encodings MUST use an appropriate character set specification to be consistent with this specification.

The complete US-ASCII character set is listed in [US-ASCII]. Note that the control characters including DEL (0-31, 127) have no defined meaning apart from the combination CRLF (ASCII values 13 and 10) indicating a new line. Two of the characters have de facto meanings in wide use: FF (12) often means "start subsequent text on the beginning of a new page"; and TAB or HT (9) often (though not always) means "move the cursor to the next available column after the current position where the column number is a multiple of 8 (counting the first column as column 0)." Apart from this, any use of the control characters or DEL in a body must be part of a private agreement between the sender and recipient. Such private agreements are discouraged and should be replaced by the other capabilities of this document. NOTE: Beyond US-ASCII, an enormous proliferation of character sets is possible. It is the opinion of the IETF working group that a large number of character sets is NOT a good thing. We would prefer to specify a single character set that can be used universally for representing all of the world's languages in electronic mail. Unfortunately, existing practice in several communities seems to point to the continued use of multiple character sets in the near future. For this reason, we define names for a small number of character sets for which a strong constituent base exists. It is our hope that ISO 10646 or some other effort will eventually define a single world character set which can then be specified for use in Internet mail, but in the advance of that definition we cannot specify the use of ISO 10646, Unicode, or any other character set whose definition is, as of this writing, incomplete.

The defined charset values are:

as defined in [US-ASCII].
where "X" is to be replaced, as necessary, for the parts of ISO-8859 [ISO- 8859]. Note that the ISO 646 character sets have deliberately been omitted in favor of their 8859 replacements, which are the designated character sets for Internet mail. As of the publication of this document, the legitimate values for "X" are the digits 1 through 9.
Note that the character set used, if anything other than US-ASCII, must always be explicitly specified in the Content-Type field.

No other character set name may be used in Internet mail without the publication of a formal specification and its registration with IANA as described in Appendix F, or by private agreement, in which case the character set name must begin with "X-".

Implementors are discouraged from defining new character sets for mail use unless absolutely necessary.

The "charset" parameter has been defined primarily for the purpose of textual data, and is described in this section for that reason. However, it is conceivable that non- textual data might also wish to specify a charset value for some purpose, in which case the same syntax and values should be used.

In general, mail-sending software should always use the "lowest common denominator" character set possible. For example, if a body contains only US-ASCII characters, it should be marked as being in the US-ASCII character set, not ISO-8859-1, which, like all the ISO-8859 family of character sets, is a superset of US-ASCII. More generally, if a widely-used character set is a subset of another character set, and a body contains only characters in the widely-used subset, it should be labeled as being in that subset. This will increase the chances that the recipient will be able to view the mail correctly.

7.1.2 The Text/plain subtype

The primary subtype of text is "plain". This indicates plain (unformatted) text. The default Content-Type for Internet mail, "text/plain; charset=us-ascii", describes existing Internet practice, that is, it is the type of body defined by RFC 822.

7.1.3 The Text/richtext subtype

In order to promote the wider interoperability of simple formatted text, this document defines an extremely simple subtype of "text", the "richtext" subtype. This subtype was designed to meet the following criteria: