AbstractThe present paper provides a methodological case study on how underlying incipient grammar change might be discerned even when frequencies of the incoming variant are apparently marginal and stable. Analysing the spread of tonic possessive pronouns in complements of locative adverbial constructions in European Spanish from a probabilistic perspective, more than 11,000 locative constructions from 1900 to 2004 were compiled, and probabilistic grammar change was operationalised as an interactive function between language-internal predictors and real time. The results reveal that numerous intralinguistic factors have been and are active in constraining the variation, with the innovation spreading significantly in spite of apparent stability in frequency. Crucially, the findings demonstrate that, even in a relatively standardised written language where the innovation has a considerably low frequency, the innovation grammaticalises along the same pathway as in colloquial vernaculars where the incoming variant is employed much more frequently.