The Dexter Hypertext Reference Model
The Dexter reference model resulted from workshops on hypertext
that were organized by Jan Walker and John Leggett. They brought
together a group of experienced hypertext system designers. The
first workshop was held in October 1988, at the Dexter Inn in New
Hampshire. These workshops were primarily funded by Digital Equipment
Corporation and Texas A&M University.
The goal was to hold discussion that might lead to a reference
hypertext system model. After several meetings, they successfully
developed a reference model, which is called Dexter Hypertext Reference
The Dexter Hypertext Reference Model is "an attempt to
capture, both formally and informally, the important abstractions
found in a wide range of existing hypertext systems and future hypertext
systems" [Halasz and Schwartz, 1994].
The Dexter reference model can serves as a starting point for the
design of hypertext system, as well as a principle basis for comparing
system and for developing standards in interchange and interoperability.
Instead of starting from scratch, a designer can start building
a hypertext system by following the constructs shown in the Dexter
Many models have evolved to refine, formalize and find a common
abstraction in hypertext systems. However, the Dexter model is the
most complete and general formal model of hypertext systems. The
model defines standard hypertext terminology and specifies common
abstractions in different hypertext systems. It tries to get the
important abstractions found in the wide range of existing and future
The model is divided in three layers, as shown in the figure below:
Source: Halasz, F., Schwartz,
M., The Dexter Hypertext Reference Model, NIST Hypertext Standardization
Workshop, February 1990.
The Dexter model focuses on the storage
layer. This layer describes how the hypertext components and
links are connected together. The basic unit in this layer is component
which is used as a container for data storage. The storage layer
acts as a "database" that is composed of a hierarchy of
data-containing components/nodes which are interconnected by links.
The components contain the chunks of text, graphics, images, animations,
etc. that form the basic content in the hypertext network.
The storage layer focuses on the mechanisms by which
the components and links are "glued together" to form
hypertext networks. The components are treated in this layer as
generic containers of data. Each component
contains sets of attributes, presentation specification, anchors,
and content specification.
The within-component layer is responsible
for the content selection of individual component through anchors
and structure within the components of the hypertext network. The
range of possible content/structure that can be included in a component
is open-ended. Content selection means the selection of a portion
of the content. If the content of a hypertext component is a file,
the ability to locate and display only the second line of this file
is an example of content selection.
The within component layer is purposefully not elaborated within
the Dexter model. The Dexter model treats within-component structure
as being outside of the hypertext model intrinsically. A mechanism
for referring to locations or items within the content of an individual
component is called anchoring.
The functionality of the layer is acting as tools for the user
to access, view and manipulate the network structure, because the
storage and within-component layers treat hypertext as an essentially
passive data structure. The model doesn't say much in this layer
because there are so many possibilities for handling user interface.
The Dexter model uses presentation specifications
as the interface between the storage layer and the run-time layer.
Presentation specifications contain instructions on how the component
is displayed. The specifications are a mechanism by which information
about how a component/network is to be presented to the user can
be encoded into the hypertext network at the storage layer.