This is perhaps a dumb question, but why is Geoffroi de Charny called "Charny" rather than "Geoffroi" when we're just using one name? Most of the time, the "de" is more of a designation than part of the name, so we just use their proper name, such as in Christine de Pizan, who would normally just be called "Christine."* The case of Joan d'Arc is an odd one since "d'Arc" is actually her name -- she's from a town called Domremy, and as far as I know there isn't even a place called "Arc," so "Joan of Arc" is a misnomer.
But I notice that the practice is to call Geoffroi de Charny "Charny," and I also note that it's the way he signs his own book. So, does anyone out there know why he's called "Charny" rather than "Geoffroi"?
*Though this too is a little complicated, since she's from Venice, and "Pizan" is an inherited name indicating her family's origins in Pizzano.