In 1850, during its first year of publication, Notes and Queries printed the enquiry of one ‘J.M.’, who asked for information about a treatise of equivocation to which he had found reference in a seventeenth-century book. A number of replies in subsequent issues provided bits of information about the treatise, and called attention to a manuscript copy in the Bodleian. This flurry of items in Notes and Queries seems to have prompted David Jardine (1794-1860), who later wrote a history of the Gunpowder Plot, to prepare an edition of the treatise, which was published in 1851. Jardine composed a preface, but since he could not establish who had written the work, and was not optimistic that the matter would ever be clarified, his account is far from complete. There did, however, exist documents that could identify the author, and exactly 100 years after the appearance of Jardine's edition, an article by A. F. Allison showed conclusively that the treatise had been written by Father Henry Garnet, S.J. Since the treatise played a significant part in the history of ‘equivocation’ in England, and was a subject of controversy at the time of the Gunpowder Plot, I believe it will be useful to provide an account of what is now known about the work.