Based on a Relevance Theory-informed view of language development, this paper argues that grammatical relations are construction-specific conventionalizations (grammaticalizations) of implicatures which arise out of repeated patterns of reference to particular types of referents. Once conventionalized, these structures function to constrain the hearer's identification of referents in discourse. As they are construction-specific, and hence language-specific, there is no category "subject" across languages; different languages will either show this type of grammaticalization or not, and if they do, may show it or not in different constructions. Any cross-linguistic use of terms such as "subject" (and "S", as in "SOV") should then be avoided.