ABSTRACT This paper discusses the use of zero and that as connectors for object clauses in eighteenth-century English. Previous approaches have shown that the use of zero increased in late Middle English and became well-established in the second half of the sixteenth century, especially in speech-based text-types. Our own research, based on the evidence of The Parsed Corpus of Early English Correspondence, has shown that in the mid-seventeenth century zero was well beyond the mid-range stage of lexical diffusion (73%) with the five high frequency verbs to know, to think, to say, to tell and to hope. Chronologically, it has been shown that progress of the zero innovation was thwarted in the eighteenth century, which has been attributed to the effect of prescriptivism. In this paper, we intend to throw light on this issue by tracking the courses of zero and that during the eighteenth century in both usage and precept. With this purpose we rely on two sources of information. On the one hand, a precept corpus containing a selection of grammars published in Britain in 1681–1800. Usage, on the other, has been surveyed in light of the evidence of The Corpus of Early English Correspondence Extension Sampler. Our aim is to assess the existence of a correlation between usage and precept in the expression of object clauses in this period with the following objectives: i) to analyse the use and distribution of zero and that clauses among grammarians; ii) to evaluate the incidence of the phenomenon in correspondence, as an input mirroring the vernacular; and iii) to assess the role of the grammarians’ statements in the propagation of the marked conjunction over the period.