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The original ASCII character code, which provides 128 different characters, numbered 0 to 127. ASCII and 7-bit ASCII are synonymous. Since the 8-bit byte is the common storage element, ASCII leaves room for 128 additional characters, which are used to represent a host of foreign language and other symbols (see code page). If none of the additional character combinations is used (128-255), the first bit of the byte is 0.

Squeeze Into Seven Bits
Early Internet mail systems as well as certain PBXs support only 7-bit ASCII codes. In order to transmit executable programs and multimedia files over such systems, these files, which use all eight bits of the byte, must be turned into a 7-bit format using such encoding methods as MIME, UUcoding and BinHex. This is why e-mail attachments are larger than their original file size. Their 8-bit data have been converted to 7-bit characters, which adds extra bytes to encode them. See 8-bit ASCII and ASCII.