https://en.m.wikipedia.org/wiki/Seventh_son_of_a_seventh_son

I was making a Terry Pratchett Discworld reference, but Terry was making a reference to the belief here on Roundworld.

On the Disc it’s the 8th son of an 8th son (“8” being the more magical number, along with the eighth color only wizards can see, octarine). The 8th son of an 8th son of an 8th son is a sourcerer, but they’re rare (on purpose).