# Org is implemented on top of Outline mode

Org manual states in its beginning

Org is implemented on top of Outline mode, which makes it possible to keep the content of large files well structured.

I referenced 'outline-mode' by C-h f outline-mode, and curious about :

Is outline-mode is part of org, which was written by org team?
or it a emacs-builtin before Org?

How could I check it?

One option is to check the source.

M-x find-library outline

;; Copyright (C) 1986, 1993-1995, 1997, 2000-2019 Free Software Foundation, Inc.

;; This file is part of GNU Emacs.

M-x find-library org

;; Copyright (C) 2004-2019 Free Software Foundation, Inc.

;; This file is part of GNU Emacs.

So, both are included with Emacs but outline-mode predates org-mode.

Poking further you can see that org-mode is actually implemented as a derived mode of outline:

(define-derived-mode org-mode outline-mode "Org" ...

• Another important point: outline is a single file with ~1000 lines of code. org is > 150000 lines of code in 200 files. So org started as an extension of outline, but the outline part is a very small piece of the whole system. – Tyler Jan 14 '20 at 17:12