RFC 1521:MIME (Multipurpose Internet Mail Extensio...
RFC-Ref

high-order bit


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... 8bit" means that the lines are short, but there may be non-ASCII characters (octets with the high-order bit set). "Binary" means that not only may non-ASCII ...
... That is, the first bit in the stream will be the high-order bit in the first byte, and the eighth bit will be the low-order bit ...



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