Alternation among restrictive relativizers in written Standard English is undergoing a massive shift from which to that. In corpora of written-edited-published British and American English covering the period from 1961–1992, American English spearheads this change. We study 16,868 restrictive relative clauses with inanimate antecedents from the Brown quartet of corpora. Predictors include additional areas of variation regulated by prescriptivism. We show that: (i) relativizer deletion follows different constraints from the selection of either that or which; (ii) this change is a case of institutionally backed colloquialization-cum-Americanization; and (iii) uptake of the precept correlates with avoidance of the passive voice at the text level but not with other prescriptive rules.