What is appledouble?

Some articles on appledouble:

Apple Single And Apple Double Formats
... AppleSingle Format and AppleDouble Format are file formats developed by Apple Computer to store Mac OS "dual-forked" files on the Unix filesystem being used in A/UX, Apple's first Unix-like ... information into a single file, whereas AppleDouble stored them as two separate files ... AppleDouble leaves the data fork in its original format, allowing it to be edited by normal Unix utilities ...
Resource Fork - Compatibility Problems
... In those cases, OS X stores metadata and resource forks using a technique called AppleDouble, in which the data fork is written as one file, and the resource fork and metadata are written as an ... will (by default) ignore ADSes and use AppleDouble format to handle forks ... a fileserver supports both AFP and NFS, then clients using NFS will store files in AppleDouble format, whereas AFP users will use AppleSingle format (resource forks are stored natively) ...