Figure shows an overview of the "Triangle Model of Activity'', which is the basis of our activity modelling approach. This is a variant of earlier versionsthat expressed in  and .
The vertical dimension reflects action decomposition, the horizontal dimension reflects time. Inputs and outputs are split into three principal categories (authority, conditions and resources). However, depending on need, arbitrarily complex modelling is possible in all dimensions.
Figure 1: O-Plan Triangle model of Activity
There is a deliberate and direct mapping between the O-Plan triangle model of activity and the existing IDEF methodology  (see figure ). IDEF modellers usually use "control'' for authority related triggers and "mechanism'' to reflect resource availability.
Figure 2: IDEF-0 model
In R-Charts  there is a visual representation of software "design units'' (modules or functions) which bears some resemblance to our activity model.