The Ciphers of the Monks: A Forgotten Number-notation of the Middle Ages is a book by David A. King published in 2001. It describes the Cistercian numeral system, a numeral system that was used by the Cistercian order of monks in the 13th to 15th centuries of the Middle Ages, and has been used sporadically since then. It allowed writing numbers from 1 to 9999 as single compound characters.[1]