Transporting large quantities information has always been a challenge, including when that information was astrological tables and your medium was vellum.

"Astrological man," late-14th-century folding almanac (Wellcome Trust)

Nowadays we are used to having a wealth of information at our fingertips. Nobody has to remember anything anymore, it is widely observed, because you can just look it all up the instant you need to know something.

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But memory has always been a faulty mechanism, and humans have often needed informational assistants (Freud called them "mnemic apparati") to go about their business.

For medieval physicians, the mnemic apparatus of choice was what is sometimes today known as a folding almanac or a belt book. There are thought to be just 29 such almanacs that have survived to the present day.

The almanacs contained detailed astrological calendars, lunar tables, diagrams of the human body and so on necessary for the practice of lunar medicine during the 14th and 15th centuries. They were small and strung onto a cord that attached to a physcian's girdle or belt.

Rosenbach Museum and Library

Each page was folded in a way that allowed it to expand to a larger format. (For more on how that worked, checked out this delightful blog post by someone who attempted to recreate one of these almanacs.)

British Library

Hilary M. Carey of the University of Newcastle has argued that the portability of so much information allowed for a more complex astrological medicine to rise in England at this time. She writes:

The folded almanac provided the texts, tables, and diagrams required by any practitioner or patient who wished to consider the planetary hours and the place of the moon in the signs before undertaking a medical procedure. This level of practice constituted the most elementary form of medical astrology. Nevertheless, it was a considerable intellectual advance on the more popular regime of noting good and evil days of the month.

Still a long way to go to an iPhone -- or even a pocket dictionary -- but in its form we recognize its purpose, so familiar to us. As essayist Katherine A. Powers put it for Boston College's magazine, "How modern it is in its aura of utility! It speaks of autonomy and independence, and evokes the individual and mobility, the busy, busy man of the world."

H/t @wynkenhimself