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binary file

A computer file format in which each character position can hold any one of 256 different binary codes. Binary files are contrasted to "ASCII files," which contain a smaller set of codes for only alphabetic letters, numeric digits, special symbols and a few control codes. Binary files can contain ASCII and many more codes because they use all eight bits of the byte, whereas ASCII files use only seven bits, which limits the possible combinations.

Executable software (machine language programs), most word processing files and database, spreadsheet and multimedia files are binary files. However, text and source program files as well as HTML and XML files are ASCII text files, not binary.

Attachments Must Be Encoded
The binary vs. ASCII distinction is made when attaching files via email. The Internet's SMTP mail protocol supports only ASCII. When binary files are attached to email messages, their 8-bit format is converted into a temporary 7-bit format to pass through mail servers, especially older ones. Encoding formats such as MIME, UUcoding and BinHex are used, and at the receiving end, they convert the 7-bit code back into 8-bit binary files. The 8-bit to 7-bit conversion makes attached files larger as they traverse the Internet. See binary, byte and ASCII.

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