In cognitive grammar, Ronald Langacker proposes a network model in order to account for the structure of grammatical categories. In this model, members of a category are viewed as nodes in a complex network, analogous to the notion of the radial category associated with George Lakoff in the development of cognitive lexical semantics. In the network model, the links between nodes in a network arise from a number of different kinds of categorising relationships that hold between the symbolic assemblies stored in the grammatical inventory. One categorising relationship is extension from a prototype, represented as (A) →(B), where A is the prototype and B shares some but not all attributes of A and is thus categorised as an instance of that category. A second type of categorising relationship is schema-instance organization, represented as (A) →(B). The schema structures those symbolic assemblies related to it as a category within the network, and novel expressions can be compared against such categories. The network grows 'upwards' via schematization 'outwards' via extension and 'downwards' as more detailed instances are added.