This article is from the Mac communications FAQ, by Bruce L Grubb BruceG6069@aol.com with numerous contributions by others.
These are all Mac binary encoding (see [2.2]) formats.
a) AppleSingle and AppleDouble were developed out of a need to share
Mac file between the MacOS and A/UX (Apple's first UnixOS) as well
as allowing A/UX users to edit MacOS files. The specs of these
formats can be found at
AppleDouble is useful today because it divides a Mac file into
two files: one for the data fork (with original filename) and the
other for resource fork (with '%' prefixing the original filename)
This made it easy to adopt AppleDouble to MIME - have non-mac
systems simply ignore the '%' file.
Mac e-mail programs that use AppleSingle and AppleDouble encode them
b) MacBinary is the Mac's standard binary encoding (see [2.2]) format.
MacBinary's purpose is to encapsulate *all* information (including
the filename, creation and modification dates, file type and creator)
contained in a Macintosh file for transport over a non-Macintosh medium.
Although a Macintosh program (called MacBinary) does exist
to do the converting to and from MacBinary, almost all modern
Macintosh telecommunications and Internet programs have the
capability of converting and unconverting MacBinary files for
Dennis Brothers, Yves Lempereur, and others gathered on
CompuServe to discuss what eventually became the original
MacBinary standard. According to Lempereur, "We finally
agreed on using the MacTerminal format (without the modified
XModem protocol). I then wrote BinHex 5.0 (see [2.3]) to
support MacBinary. A year later, the same group got
together on CompuServe again and created MacBinary II."
MacBinary I is the name given to the old MacBinary standard.
MacBinary II is the name given to the c1987 update to the
MacBinary III is an update to the vernerable c1987 format
that supports the icon badge custom routing information
finder flags that are part of MacOS 8.5 and later.
Since then, BinHex and the MacBinary II have become the standard way
of encapsulating Macintosh files for transfer over foreign systems
throughout the Internet, USENET, and elsewhere. Of course with the
coming of a data only .sit format and programs like MacLHA, ZipIt,
and DropZip, StuffIt Deluxe using MacBinary internally for .lhz
and .zip PC formats MacBinary (which was never as popular as
Binhex to begin with) has been religated mainly to older
compression formats, sea, and smi files.
MacBinary's correct MIME type is "application/x-macbinary" and if
you want StuffIt Expander to launch when you double click on the file
set the type and creator fields to BINA and SITx.