The names John and Jacob are not interchangeable. However, among the PA German families prior to the mid-19th century, the males were often given the name of John plus something else (e.g. John Jacob, John George, John Frederick) where the second name was the one they really used. So a man named John Jacob would have been called just "Jacob" even though his full name was "John Jacob". This causes confusion among some researchers who are used to the modern-day practice of referring to someone by their first name rather than their middle name. I have seen the names Jacob and James used interchangeably. Tom