MacBinaryencoded

MacBinary encoding is a historical method used to store a classic Mac OS file’s data fork, resource fork, and metadata in a single binary file. It was designed to facilitate the transfer of Mac files over media or networks that did not preserve the fork structure, such as email attachments or non-Mac archives.

A MacBinary-encoded file begins with a fixed header that encodes key metadata, including the file name, file type and creator codes, Finder flags, and the lengths of the data fork and the resource fork. The header is followed by the actual bytes of the data fork and then the resource fork. This packaging allows a single file to convey all essential Mac file information and content, enabling reconstruction of the original file on a compatible system.

There were two primary specifications in common use: MacBinary I and MacBinary II, with II becoming the de facto standard for widely distributed Mac files. Some later variations introduced additional attributes or compatibility refinements, but MacBinary II remained the most widely supported version for many years.

Usage and legacy: MacBinary was popular for distributing Mac software and documents in environments that could not handle multiple forks. Decoding was required to restore the original data and resource forks and to apply the correct metadata (type, creator, and Finder settings). Today, MacBinary is largely obsolete and has been superseded by newer formats and by modern macOS handling of file metadata. It can still appear in vintage archives and requires a dedicated decoder to convert back to usable Mac files.

See also: BinHex, AppleSingle, AppleDouble.