The history of outliner software is a seldom explored subject within computer science. Early examples were created in the late 1980s as hypertext editors. A notable example was the Symbolics Concordia software which has much in common with the later Windows 3.1 help files. The idea is that the user creates a hierarchical tree and can insert hyperlinks for easier navigation. [1]
Sure from a technical perspective, there was the hypercard program which was published for the Apple computer earlier. The problem with this attempt was, that in the early 1980s the homecomputers were not equipped with harddrives. For creating a larger hypertext document is harddrive is needed because the filesize will become 500 kb and more. So it makes sense to assume that the first examples of working hypertext authoring tools were created in the late 1980s on dedicated workstation computers.[4]
In the year 1995 Henk Hagedoorn created for the Windows operating system the Treepad software.[2] It is the most important software in the history of outliner programs. It was the first mainstream program which was using a two pane editor. Left the user sees the structure of the document and in the right pane the full text is shown.
From a more critical perspective, the Treepad software looks like MS-Word in which the document outline was activated. So it is basically a word processing software with a focus on the structure of the document.
After Treepad was created many clones were programmed like Keynote, cherrytree and zimwiki. Technical the difference is low or even not there. The GUI has the same two pane structure and the only improvement of the keynote program was, that it was released as open source software.
An interesting but niche software are zkn3 and synapsen. These tools were created with the attempt to emulate a Niklas Luhmann Zettelkasten. They are used by scientists for maintaining annotated bibliographies. [3]
Another more recent approach is the Obsidian software which combines a chaotic graph structure with a powerful search feature.
There is a seldom analyzed common feature between the early outliner programs and more recent note taking apps. All these programs are using a key value structure for storing information. The key is shown in the left pane which is the name of a file or the name of a section. In the right pane the value is shown which is equal to a free form text which includes images and hyperlinks.
Sources
[1] Nielsen, Jakob. Multimedia and hypertext: The Internet and beyond. Morgan Kaufmann, 1995.
[2] Interview: Henk Hagedoorn, creator of TreePad http://mediaman.com.au/interviews/hagedoorn.html
[3] Markus Krajewski synapsen – A hypertextual card index User’s Manual http://www.verzetteln.de/synapsen/_docs/SynManual.pdf
[4] Computer chronicles Personal Information Management, 1989