Active database systems enhance the functionality of traditional
databases through the use of active rules or `triggers'.  There is little
consensus, though, on what components should be included in a rule system. 
In this paper the expressive power of some simple active database rule systems
is examined and  the effect of  choosing different features studied.
Four important parameters of variation are presented, namely the rule language,
the external query language, the meta rule language and the pending rule 
structure.  We show that each of these is highly influential in determining
the expressiveness of the rule system as a whole, and that an appreciation
of them can serve as a basis for understanding the broader picture of system 
behaviour.