Proper noun
US-ASCII
(computing, formal, retronym) Synonym of ASCII, especially to differentiate it from 8-bit extended ASCII.
According to RFC 1428, the original RFC 821 definition of SMTP limits Internet Mail to lines (1000 characters or less) of 7-bit US-ASCII characters. Source: Internet
RFC 1428: "SMTP as defined in RFC 821 limits the sending of Internet Mail to US-ASCII characters." Source: Internet
Designed to be efficient and mostly human readable when used for text data consisting primarily of US-ASCII characters but also containing a small proportion of bytes with values outside that range. Source: Internet
Later the format of email messages was re-defined in order to support messages that are not entirely US-ASCII text (text messages in character sets other than US-ASCII, and non-text messages, such as audio and images). Source: Internet
The first 128 characters (US-ASCII) need one byte. Source: Internet