We get occasional calls from Mac-based corporate creative teams, asking about strange files their Windows-based co-workers are seeing on shared file servers. The files have familiar names, but all start with a “._” prefix.

Here is the explanation of where those oddly named files come from, and why they are important to the Mac-based creative team.

Before Mac OS X, most Mac files had two parts, a data fork and a resource fork. Problems could arise when saving Mac files onto non-Mac-formated disks. Resource forks were often ignored or lost in the transfer and files would end up damaged and unusable.

Today, when Mac OS X copies files onto a non-Mac file server it creates the resource fork file as a separate file with a “._” prefix. For example, copying a forked file named “examplefile.psd” to a shared server might create two similarly named files:

._examplefile.psd — the resource fork.
examplefile.psd — the main document file.

To Mac users these “._” files are hidden, but for some PC users (and other OS’s, such as Linux) these files will show up as visible.

Mac OS X does not rely on resource forks as heavily as previous versions of the Mac OS. However some applications continue to store important or crucial information in these files, so they can’t be thrown away casually.

Tip: Postscript fonts, for example, store almost all their Postscript information in the resource fork. It is common for large font collections to lose these crucial resource forks and become unusable when improperly moved between two non-Mac servers.

While the solution is not always elegant, Windows users can hide these mystery files easily enough by selecting “Do not show hidden files and folders” in Window’s Folder Options.

Source: Initial versions of this tip written by Creativetechs consultant Jasson Lewellen after encountering this issue at a couple Seattle-area inhouse creative departments. Includes information from Apple’s KBase article 106510.