Quoted-Printable Encoding
{{MIME}} content-transfer-encoding that leaves printable ASCII bytes as-is and escapes everything else as =XX hex pairs, used for mostly-Latin text with occasional non-ASCII characters so the encoded form remains roughly human-readable.
Quoted-Printable is the second of the two content-transfer-encodings defined in RFC 2045, alongside Base64. Where Base64 is dense and opaque, Quoted-Printable is sparse and mostly readable: bytes that are already printable 7-bit ASCII pass through unchanged, and only bytes outside that range — plus the literal "=" sign and a few other reserved characters — are escaped as a three-character sequence "=XX" where XX is two uppercase hexadecimal digits encoding the byte's value. For example, an ASCII form feed (decimal 12) becomes =0C, and a literal equals sign must be encoded as =3D because the "=" character is the escape introducer. Lines are limited to 76 characters; if a logical line is longer, the encoder inserts a soft line break consisting of a trailing "=" at end of line that the decoder removes. Trailing whitespace on a line must be stripped during decoding, because mail relays are notorious for adding or removing trailing spaces. Quoted-Printable is the natural choice for European text in UTF-8 or ISO-8859-x, where most bytes are still ASCII letters and only accented characters need escaping. It is a poor fit for binary data or for scripts where most bytes are non-ASCII (such as CJK languages), because the 3x expansion of every escaped byte quickly becomes worse than Base64's flat 33% overhead. The related RFC 2047 encoded-word syntax uses a Quoted-Printable variant called "Q encoding" inside non-ASCII header values.