ISO-2022-JP is a character encoding designed for sending Japanese text in email. It uses escape sequences to switch between ASCII and JIS X 0208 character sets within the same byte stream.

In the early days of the internet, many email routes could only handle 7-bit ASCII. ISO-2022-JP was valued as a 7-bit clean encoding for Japanese email. Internet history books cover the evolution of email character encoding.

Modern email systems support 8-bit encoding, and UTF-8 is now the standard. However, ISO-2022-JP is still used for compatibility with older email clients.

The escape sequences are ESC $ B (start JIS X 0208) and ESC ( B (return to ASCII). Email protocol fundamentals books explain the technical details.