Base58Check is an encoding used in the Bitcoin system to represent Legacy receiving addresses and certain other data, such as extended keys, in the form of human-readable character strings. It is a variant of the Base58 system, a positional representation of base 58 designed to minimize human transcription errors. It uses a set of 58 alphanumeric characters, consisting of the digits from 1 to 9, uppercase letters from A to Z (excluding the letters I and O to avoid confusion with the digits 1 and 0), and lowercase letters from a to z (excluding the letter l to avoid confusion with the digit 1). Base58Check differs from Base58 by adding a checksum. It is represented by a reduced version of a double SHA256 hash of the original data (SHA256d or HASH256), at the end of the data encoded in Base58. When verifying, the checksum is recalculated and compared to the one that was added during encoding. If the two hashes match, the data is considered valid; otherwise, a corruption or transcription error is reported.

The use of Base58Check in Bitcoin addresses and keys offers several advantages. Firstly, it reduces human errors in transcription and reading by avoiding ambiguous characters. Secondly, it protects against typing errors by detecting and reporting errors through the checksum. Thirdly, the compact representation of data in Base58Check reduces the space required to store and share addresses and keys. The most recent receiving addresses (post-SegWit) have abandoned this Base58Check encoding for Bech32 and Bech32m encodings, which have a more advanced checksum (with BCH codes).