Today’s question is another one that probably everyone who has spent any time thinking about names has thought about at one time or another — just how exactly do you get James as a variant of the name Jacob?! The only letters they share are Ja-, which from a modern point of view is a pretty tenuous connection.
The answer is…it’s actually pretty straightforward!
The standard Latin form of the name was Jacobus, but in Italy, southern France, and Switzerland, the -b- became -m-, e.g., Jacomus, eventually giving rise to the Italian form Giacomo and the Provencal and Catalan form Jacme. Very rarely, Jacomus became James in Old French, and when that form of the name got imported into England (early 13th C examples of James in England can be found in the records of King John), it supplanted the more common Old and Middle French forms Jaques and Jacques. This is why you tend to see James only in insular contexts; over on the continent, the Langue d’Oil forms retaining -c- dominated.