The focus of Open Hypermedia Systems (OHS) is the integration of content files of arbitrary formats with hypertextual linking structures. The presented hypertext systems Intermedia, Microcosm and Hyper-G are designed with OHS principles in mind.
David Lowe and Wendy Hall define a hypermedia system as open if it conforms with the following four conditions, Hypermedia & the Web: An Engineering Approach [Lowe/Hall 99, p. 338]:
The hypertext link service should be available across the entire range of applications available on the desktop.
The hypertext service has to deal with application programs that are unaware of the link management. As a consequence of this linking data and content have to be separated, because the original data format cannot be extended to store information about links and anchors.
The link service must work across a network of heterogeneous platforms. [Ibid.]
Especially the Internet is a medium that should be utilized to connect Open Hypermedia Systems with each other. The tools and services should be available for all major operating systems.
The architecture should be such that the functionality of the system can be extended. [Ibid.]
This calls for a modular software architecture of the system with a well defined programming interface (API) for new extensions and plug-ins.
There should be no artificial distinction between author and reader. [Ibid.]
A hypertext system is considered as open, if the user is able to change and modify the linking structure on his own. This does not imply that a user is allowed to change every aspect of the document and to alter how the document appears for someone else. But a user should have full access in her private domain. Furthermore the hypermedia system has to manage user lists and access rights in order to offer a service to share user changes with other users.