Letters patent collected together on parchment rolls. Each roll is a year, though in some cases the roll is subdivided into two or more parts. Each sheet of parchment is numbered, and called a membrane: for example, the 8th or any other sheet, say of the 10th year of Henry III., is cited thus: “Pat. 10, Hen. III., m. 8.” If the document is on the back of the roll it is called dorso, and “d” is added to the citation.
Source: Dictionary of Phrase and Fable, E. Cobham Brewer, 1894