ARCHIVED: What are AppleSingle and AppleDouble?

AppleSingle and AppleDouble are formats originally developed by Apple as a means of storing Mac OS files in A/UX, Apple's first Unix implementation. Their capabilities were expanded for use by other platforms that serve Mac OS files, such as MAE and NFS. AppleSingle and AppleDouble have also been adapted as extensions to the MIME specifications, incorporating better support for Mac OS file transport into the standard.

AppleSingle is analogous to MacBinary in that the resource and data forks, as well as Finder information, are combined into a single file. This makes it possible to store and transport Mac OS files on platforms that cannot deal with dual-forked files. Mac OS documents encoded in AppleSingle and sent as MIME attachments will have a MIME content type of application/applefile and will usually be Base64 encoded.

AppleDouble is similar, except that the data fork is stored in a separate document, resulting in two files, called the header file (resource fork and Finder information) and data file (data fork). Files stored in the AppleDouble format are more accessible to non-Mac OS systems and this format is often used on text files, images, and word processor files. AppleDouble encoded files sent as MIME attachments will be listed as multipart/appledouble documents. The header file containing the resource fork will have a content type of application/applefile and will usually be Base64 encoded. The data file may or may not be encoded and its content type will depend on what kind of file it is.

Most Mac OS MIME-compliant clients will be able to correctly decode and, in the case of AppleDouble attachments, also assemble AppleSingle and AppleDouble attachments. MIME-compliant clients on other platforms have access to the data file of AppleDouble attachments.

For more information on AppleSingle and AppleDouble as MIME types, see RFC 1740 at:

  http://www.ietf.org/rfc/rfc1740.txt