Base 64 literally means a positional numbering system using a base of 64. It is the largest power of two base that can be represented using only printable ASCII characters. This has led to its use as a transfer encoding for email among other things. All well-known variants of base 64 use the characters A–Z, a–z, and 0–9 in that order for the first 62 digits but the symbols chosen for the last two digits vary considerably between different systems that use base 64.
