The paper addresses the question of the relation between transitivity parameters, as formulated by Hopper and Thompson (1980), and transitivity alternations (i.e. alternations in case-marking patterns). First, I suggest to represent the list of transitivity parameters in the form of the scale, stretching from subject-related parameters (e.g., agentivity) via verb-related (e.g., aspect) to object-oriented (e.g. affectedness). This semantic scale could then be used to predict the ‘locus’ of case alternation (i.e. which actant changes its case-marking in the course of an alternation), on the assumption that a transitivity parameter should be preferably encoded on the ‘relevant’ constituent (i.e. constituent to which it pertains). This, basically iconic ‘Relevance principle’ interacts with a structural principle, which prohibits the manipulation on case-marking of the ‘primary’ (i.e. unmarked) actant exclusively (i.e. without a diathetic shift). It is shown how interaction of these two principles can predict the cross-linguistically preferred patterns of transitivity alternations, as well as constrain co-variation between transitivity alternations and transitivity parameters.