My massive thanks go to the following people:

To Michael Hill, who encouraged me to write this book when it was just an idea and who supported me the whole way, which was long.

To Jane Palfreyman, a great friend and the best publisher a writer could hope for. For all her continuing support, enthusiasm, intelligence and wit.

To Clara Finlay, my brilliant editor, for her in-house organisational magic as well as her copyedit which showed me the way. Far from being an invisible mender, she was my stylist.

To Ali Lavau for her astute, enthusiastic and gently critical (in the best possible way) structural edit. She helped me to understand how to shape this apparently boundless material.

To Aziza Kuypers for her eagle-eyed proofreading that went way beyond proofreading. Her questions prompted me to think more deeply at several key moments in the manuscript.

I was extremely fortunate in my publisher and three editors.

To publisher Patrick Gallagher for his ongoing support and martinis.

To Peter Long for his beautiful internal design and a cover which captures the arc of my book in one image.

To Giovanni Fazzini at the Biblioteca Nazionale Marciana in Venice, who gave me invaluable help with accessing dozens of articles and books which shed light on the work of Luca Pacioli, the Scuola di Rialto and Venetian mathematics.

To my wonderful hosts at the Casa Mila in Sansepolcro, Val and Colin Stevens, who treated me like family. And to Val especially for activating her Sansepolcro networks on my behalf to gain me entrance to places I would either not have known about or not have been allowed access to, such as the Sansepolcro Biblioteca Comunale. And to the librarians at that library, for letting me look at the 1494 edition of the Summa de arithmetica, geometria, proportione et proportionalità and De divina proportione.

To Jackson and Scarlet Hill, for being such wonderful company, for making me laugh (mostly at myself, essential when writing about monks and mathematics), and for enduring my long absences, either at my desk or in my head, over the last three years.

To Gaby Naher, a great friend whose regular emails and morning coffee dates were often the only contact I had with the outside world while I wrote this book.

And once more, to my family and friends for still talking to me despite the fact that I spent the last three years locked away in my study writing about a monk.