William Oughtred’s (1574–1660) *Key of the mathematics* (1647) was a comprehensive text on arithmetic and geometry. It was the first English edition of Oughtred's Latin *Clavis mathematicae* (1631). A claim on the title page indicates that “The Rule of Compound Usury” would be considered. Today, we refer to "compound usury" as “compound interest.”

The first section of the text is devoted to the use and format of notation. This section is reviewed here in full.

*The images above are provided courtesy of Columbia University Libraries, George Arthur Plimpton Collection.*

*Editor's note:* To see images from later pages of another copy of this book (and from other works by Oughtred), see "Mathematical Treasure: Oughtred's Key of the Mathematics" here in *Convergence*.