On Wed, 22 Dec 2004 08:35:05 -0500, Dan Carlson <minutiaeman(a)st-minutiae.com> wrote: Unix (or more properly Posix) systems don't require file type extensions in the file name. The usual way to determime the file type is to "sniff" the contents of the file as well as certain directory information to derive it's mime type. The file command on posix system is a user command which does this sniffing. The traditional Mac file system(s) did more than just storing the file type separately, Mac files had two 'forks' The data fork usually corresponds to the file contents on other systems, while the resource fork is actually a small data base in which resources can be looked up by resource type and resource id. Some of those resources are the File creator which contains a code identifying the application which created the file, and the file type which indentifies the kind of file that this is for that application. So, for example a Word doc file and a Word rtf file will have the same creator but different file types. The resource fork can contain other things as well, such as an icon to override the one normally shown in the finder (which is determined by the combination of type and creator), and anything else the application wants to store there. In the case of executable files, almost everything including the executable code is actually stored in the resource fork as resources. The above is based on memory, I don't khow they are doing this under OS X.