In the context of multilingualism in later medieval Britain, the influx of French termi-nology into the emerging technical vocabulary of Middle English is likely to have pro-duced synchronous synonyms. For functional reasons, some native terms are expected to be dropped from the language, others to undergo differentiation through semantic shift. A significant proportion of the French borrowings are often seen as having been new technical terms, but earlier historical research on the nature of technical vocabulary in English has not clearly characterised this lexical domain; ways are therefore explored here of identifying technical terminology in this period. Definitions contained in histori-cal dictionaries, principally the Middle English Dictionary, provide the main diagnostic, specificity of meaning. As a case study, borrowings in a technical register are examined using the terms contained in the sub-domain ‘Instruments’ within the Middle English vocabulary for Building (extracted from the Bilingual Thesaurus of Everyday Life in Medieval England project) supplemented with lexis from the Historical Thesaurus. Uti-lizing the components of meaning in the Middle English Dictionary and the Oxford English Dictionary definitions, the lexical items are classified into semantic hierarchies as was done for the Historical Thesaurus of the Oxford English Dictionary. In addition to dates of first usage, etymological information about the lexical items is included in the semantic hierarchies, allowing analysis of patterns of replacement by borrowed terms at different levels of the lexicon. It is found that the impact of French on the na-tive lexicon in this dataset is most evident at the superordinate and basic levels of the lexicon, where we find almost equal numbers of native and borrowed terms, while at the hyponymic level native terms are in the vast majority. The study provides an insight into the vocabulary of speakers of the Middle English period with a high level of expe-rience and expertise in technical fields and the findings suggest a resistance to borrowed vocabulary not at the lowest section of the social stratum, but rather by the class of skilled workers.