At least some content in this article is derived from information featured in Harry Potter: Hogwarts Mystery. Spoilers will be present within the article.
Presumably taught by Septima Vector in Arithmancy at Hogwarts School of Witchcraft and Wizardry, books on the subject included Numerology and Grammatica and New Theory of Numerology. However, Sybill Trelawney was seen teaching Numerology in Divination class instead during the 1989–1990 school year, and claimed this was not Arithmancy, whatever Professor Vector said.
A important part of the subject was "reduction", which meant adding up the digits in the number to form a new number. It's unknown, though, whether the new number needed reducing further along if it also had more than one digit.
- Harry Potter and the Prisoner of Azkaban (First mentioned) (Mentioned on book)
- Harry Potter and the Order of the Phoenix (Mentioned on book)
- Harry Potter: Hogwarts Mystery (First appearance)