Apache Reference: mod_dir

mod_dir - Content Selection by Using Directory Default Documents
Since Apache 1.0, src/modules/standard/mod_dir.c
Rob McCool, Robert S. Thau (1993)

mod_dir performs a basic task of any web server: after some URL transformation has mapped a URL to a directory on the local file system, this module tries to select the default document inside this directory. It also solves a related problem: when the URL does not end in a slash (not xxxC) but was nevertheless mapped to a directory rather than a file, a slash is appended to the URL and a HTTP redirect is performed to avoid problems with relative hyperlinks inside the document.

Directive:

DirectoryIndex