Two different invariable relative markers were in use in early English, þe and þat/that. This paper aims to answer the question of how and why þat replaced þe as an invariable relativizer in Middle English. To this end I analyse the distribution of invariable relativizers in the relevant periods of the English language (from Old English to late Middle English) as represented in The Helsinki Corpus of English Texts. The following variables are examined: (i) the syntactic function of the relativizer, which determines the progression and recession of relativization strategies, following the Accessibility Hierarchy proposed by Keenan and Comrie (1977); (ii) the type of relative clause (whether restrictive or non-restrictive), which conditions the distribution of relativization strategies in particular; (iii) the type of NP antecedent, which also plays a role in the selection of relativizer; (iv) the text type; and (v) dialect. The analysis reveals that þat/that starts to replace þe very slowly, occupying the environments less favoured by þe, that is, those of object and resuming inanimate antecedents. Moreover, I will show that this slow, progressive introduction suddenly evolves into a dramatic change, with þat quickly becoming the only invariable relativizer available.