This study describes a change in which relative clause extraposition is in the process of being lost in English, Icelandic, French, and Portuguese. This current change in progress has never been observed before, probably because it is so slow that it is undetectable without the aid of multiple diachronic parsed corpora (treebanks) with time depths of over 500 years each. Building on insights from Kiparsky (1995), the study shows that the change may date as far back as the innovation of Proto-Germanic and Proto-Romance relative clauses, as these varieties differentiated from Proto-Indo-European. It also shows that the unusually slow speed of the change is due to partial specialization of the construction along the dimension of prosodic weight, following the argument made at greater length in Fruehwald & Wallenberg 2016. Finally, the change is shown to have important consequences for the syntax of extraposition, supporting the adjunction analysis of Culi- cover and Rochemont (1990). The article also discusses the implications of Sauerland's (2003) analysis of English relative clauses, and while modern English data supports his analysis, the diachronic extraposition data is not yet fine-grained enough to bear on the ‘raising’ analysis of relatives in general. This is identified as an important question for further research on this change.