The medieval number system was invented by the Cistercian order in Europe in the 13th century. Cistercians are a Catholic religious order of monks and nuns. They used, amongst themselves, a secret number notation. No one really knows about it until about 20 years ago, a British orientalist and historian David A. King wrote a book about it. It was used by various different groups for kinds of medieval cyphers and secret number systems. It is a system that can only write every number from 0 to 9999. It can not to any other numbers. It is really easy to use for dates and page numbers of books.
If you are interested in these numbers watch this Numberophile video.