During the Carolingian era (eighth- and ninth-century Europe), liking how a name sounded or wishing to be original were not good reasons to choose an appellation for your child. If you were an aristocrat, names were important, especially if you were an upstart family who had seized the crown in a coup.
The following men had more children than listed, but you’ll see the frustration historical novelists encounter when writing about real people: Pepin begat Charles. Charles begat Carloman and Pepin. Pepin begat Charles and Carloman. Carloman begat Pepin, and Charles begat Pepin, Charles, another Pepin (originally named Carloman), and Louis, who did his own share of begetting.
Today, in nonfiction, we call the first Charles the Hammer, the second Pepin the Short, the second Charles the Great, or more commonly Charlemagne. We distinguish Charlemagne’s sons as Pepin the Hunchback, Charles the Younger, Pepin of Italy, and Louis the Pious. The Carlomans don’t have nicknames, but they lived far enough apart to be distinguished by time period.
In a historical novel, however, the author cannot use the Hammer, the Short, or Charlemagne. The characters would not have called them that. Although they might have called Charlemagne’s son Young Charles or Little Charles, it gets cumbersome.
One solution is for a relative to call them grandfather or uncle and have another character be enough of an outsider to have to process the information. My first novel, The Cross and the Dragon, contains this example:
They stopped walking and looked out toward the Rhine, a silvery purple in the moonlight. “You know my mother grew up in Grandfather Pepin’s palaces,” he said.
King Pepin, our present king’s father, Alda reminded herself. Shall I ever become accustomed to his manner of referring to the royal family? Alda nodded for Hruodland to continue.
Another solution is to cheat a little, using variants of the name and spelling. I call Charlemagne’s son Karl. As for Charlemagne’s two Pepins, I am fortunate enough that my first two books take place before his renaming, which allows me to call him Carloman. In the third book, Queen of the Darkest Hour, I have a Pepin and Pippin.