The precise choice of characters is difficult. The earliest instances of this type of encoding were created for dialup communication between systems running the same OS – e.g. Uuencode for UNIX, BinHex for the TRS-80 (later adapted for the Macintosh) – and could therefore make more assumptions about what characters were safe to use. For instance, Uuencode uses uppercase letters, digits, and many punctuation characters, but no lowercase, since UNIX was sometimes used with terminals that did not support distinct letter case. Unfortunately for interoperability with non-UNIX systems, some of the punctuation characters do not exist in other traditional character sets. The MIME Base64 encoding replaces most of the punctuation characters with the lowercase letters, a reasonable requirement by the time it was designed.
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