Despite being a self-confessed math-phobic J.K. Rowling has certainly had some fun with numbers in the Harry Potter universe. In a lexicon.org/2018/02/24/wizard-currency/">previous episode we talked about wizard currency being built on prime numbers, with 29 knuts to a sickle and 17 sickles to a galleon. Prime numbers crop up elsewhere in the series too. The most obvious is the number lexicon.org/2005/08/20/the-number-seven/">seven, described as the most powerfully magical number of all. Seven books, seven years, seven Horcruxes, seven Weasley children, Harry’s wand cost seven galleons, and many more examples. Thirteen also features, most notably when Professor Trelawney describes the ill omen of having thirteen dine together for Christmas dinner in Prisoner of Azkaban. Put 7 and 13 together and you have the number of the lexicon.org/place/great-britain/england/london/diagon-alley/gringotts-wizarding-bank/gringotts-vaults/vault-713/">vault in lexicon.org/place/great-britain/england/london/diagon-alley/gringotts-wizarding-bank/">Gringotts from which Hagrid retrieved the lexicon.org/thing/philosophers-stone/">Philosopher’s Stone. Seven and thirteen were also the directions Dumbledore gave to Harry and Hermione for rescuing Sirius and Buckbeak – 7th floor, 13th window. Three is another prime number which features extensively – the trio of Harry, Ron and Hermione, three lexicon.org/2007/07/22/deathly-hallows-2/">Hallows, three types of wand core, three lexicon.org/magic/unforgivable-curses/">Unforgivable Curses. Students receive their Hogwarts letter on their eleventh birthday, and the Hogwarts Express leaves at 11 o’clock – another prime number. Wizards come of age on their 17th birthday, and the Epilogue is set 19 years later – yet more prime numbers. Could this preponderance of prime numbers be a coincidence? I doubt it.