This paper explores degrees of equivalence constructed in English technical definitions. From an ergative perspective, the degree of equivalence depends upon the effectiveness of the clause (i.e. effective vs middle). Effective clauses tend to be encoding in orientation (i.e. the direction of the coding in the clause is from the gloss to the term) whereas middle clauses tend to be decoding (from term to gloss) (Halliday 1967/8, 1994; Davidse 1992a, 1996). In technical definitions, ideational meaning (expressed in relational clauses) is typically metafunctionally dominant and lexicogrammatically "deautomatized" (Mukar‡ovsky 1977), while interpersonal meanings (expressed in mood and modality) are less dominant and "automatized". In technical definitions, the source of knowledge is either absent but probeable or absent and non-probeable. It is argued here that, despite the general automatization of interpersonal meaning in definitional clauses, a residue of interpersonality is in fact critical to the degree of equivalence constructed in the clause.