What is the folded almanac?: the form and function of a key manuscript source for astro-medical practice in later medieval England

This article provides an account of an important source for late medieval English medicine and astrology, namely the folded almanac, also termed, less correctly, a physician's folded (or folding) calendar, vade mecum, or girdle book. On the basis of a discussion of twenty-nine surviving examples of the folded almanac, a number of questions are raised about the way these manuscripts were used in astro-medical practice. This article reviews what this group of manuscripts should be called, what kind of manuscript they are, and assesses the way in which they were worn on the body. It establishes that the folded almanac was an English innovation and that many include a new, updated version of the calendar and lunar data of John Somer. A subsequent article will describe the working components of the folded almanac, and the way in which they were used in the practice of astrological medicine.