MH (and its front ends) take advantage of the UNIX filesystem structure for storing folders, messages, and other files. As a new user, you really don't need to understand this because MH effectively hides the filesystem from you. You may want to skip this chapter.
If you're going very far with MH, though (especially if you're programming with it), you'll want to understand how MH stores messages and folders. You'll also need to know where other MH files are kept.
The chapter starts with a diagram of some directories and files that are important to MH. As you read about MH, you might want to refer to this diagram. Next are explanations of specific MH files. Finally, there's a warning about the format of some MH files.
To read this chapter, you'll need to know the basics of UNIX filesystems: pathnames, directories, and subdirectories, as shown in the Figure Important parts of a UNIX filesystem.