The aim of this paper is to explore reasoning in SFL lexicogrammatical descriptions by focusing on interpersonal clause types organised in the system of mood. To begin, system-structure relations embodied by the theoretical dimension of axis are introduced in relation to the description of imperative, declarative and interrogative clauses in English. The paper then moves to a three-fold perspective on mood distinctions, captured in SFL by the ‘trinocular principle’: interpersonal clause types are first looked at in terms of their contribution to the dialogic negotiation ‘from above’; they are then approached in terms of the paradigmatic environment they define ‘from around’ in close relation to the structural patterns motivating paradigmatic choices ‘from below’. English mood is reconsidered along these lines, and then a different language is used as an illustrative example for the reasoning explored: Spanish. Finally, the paper addresses the implications of the exploration proposed for the description of interpersonal lexicogrammar in Spanish and, more generally, for SFL descriptive work across languages.