AppleDouble

AppleDouble is one of the systems used to store the Resource Fork of Macintosh files on filesystems not natively supporting it, something which became necessary when Apple moved to Unix-based operating systems instead of "classic" MacOS. AppleSingle is an alternative format to accomplish the same end, combining all the forks plus a metadata header in one file instead of keeping separate files like AppleDouble.

The AppleDouble format keeps the data fork of the file in its original format and filename (this is the main file, as used by non-Mac operating systems, and for many file formats, the only one that matters), and creates a second file with the resource fork as well as Finder metadata. The second file has the filename of the main file with "._" (a dot and an underscore) preceding it. If it was encoded for transmission, Base64 was generally used.

Format detail links

 * File format details
 * RFC 1740 (describes various Mac-specific formats)

Utilities

 * Mac Binary Converter
 * Mac::AppleSingleDouble Perl library

Other links

 * Wikipedia article
 * Forensics Wiki article
 * MacBinary and Binhex 4.0 considered harmful
 * Apple KnowledgeBase notes about AppleDouble