Two men each marry the other's sister. What are their sons to each other?
In one of Alcuin of York's 8th-century arithmetic riddles for students, two men marry each other's sisters. Years later, each man has a son. Alcuin's question: exactly what family relationship do the two boys have to one another?
Reveal the answer
They are double first cousins, related twice over, since each boy's father is also the other boy's maternal uncle. It's one of the oldest surviving family-relationship logic riddles, from Propositiones ad Acuendos Juvenes ('Problems to Sharpen the Young'), attributed to Alcuin of York (c. 735-804), tutor to Charlemagne.
— Alcuin of York, Propositiones ad Acuendos Juvenes — c. 800 AD