The use of the word cousin as a term of address for non-relatives in late-medieval and Renaissance English is well documented in letters between monarchs, but weak for other social groups in the standard dictionaries, with one example each in the Oxford English Dictionary and the Middle English Dictionary. As it is difficult to establish for earlier periods whether people were blood relations, an investigation of cousin as a term of address needs to establish the relationship between addressor and addressee, as far as possible, from independent historical sources. This study is based on the use of the term cousin in letters, as this often provides precise information on the relationships of correspondents. This investigation documents the use of cousin from the thirteenth to the early-sixteenth century in all literate ranks of society and concludes that the royal use of cousin constitutes a relic of an earlier more widespread use.