Initially, the legal values for "encoding" are "Q" and "B". These
encodings are described below. The "Q" encoding is recommended for
use when most of the characters to be encoded are in the ASCII
character set; otherwise, the "B" encoding should be used.
Nevertheless, a mail reader which claims to recognize encoded-words
MUST be able to accept either encoding for any character set which it
Only a subset of the printable ASCII characters may be used in
encoded-text. Space and tab characters are not allowed, so that the
beginning and end of an encoded-word are obvious. The "?" character
is used within an encoded-word to separate the various portions of
the encoded-word from one another, and thus cannot appear in the
encoded-text portion. Other characters are also illegal in certain
contexts. For example, an encoded-word in a "phrase" preceding an
address in a From header field may not contain any of the "specials"
defined in RFC 822. Finally, certain other characters are disallowed
in some contexts, to ensure reliability for messages that pass
through internetwork mail gateways.
The "B" encoding automatically meets these requirements. The "Q"
encoding allows a wide range of printable characters to be used in
non-critical locations in the message header (e.g., Subject), with
fewer characters available for use in other locations.