When you set a window background pattern, it's stored as a small invisible file in that folder. (Only one file is stored for that folder and all subfolders; there isn't a separate file for each folder. The file is later deleted if you choose "None" as the background pattern.)

This method is better than storing this information in the preferences file, because it means that the backgrounds can be shared with others.

For example, if someone sets a pattern on a server folder, all Window Monkey users on the network see the same pattern. And if you compress a folder with a Window Monkey background pattern and send it to another person who has Window Monkey, they'll see the same pattern when they decompress it.

In fact, this is exactly the same way your Macintosh already handles custom folder icons, and for the same reasons. Whenever someone sends you a folder with a custom icon, it also has a small invisible file in the folder. We just copied Apple's good idea.

Some people think that any invisible file must mean there's an evil conspiracy afoot, or a virus on their disk. However, our files — named "Background Picture" plus a carriage return — are simply normal Macintosh PICT files; if you make them visible using ResEdit, FileTyper, or a similar utility, you can open them in any painting program. They're quite harmless, and we make them invisible only because it would be annoying to see them in your folders.