The couple originally named their newborn X Æ A-12. However, that name violated California state law which only permits the 26 letters from the alphabet for baby names, prompting the duo to change the name to X Æ A-Xii.
Grimes, born Claire Elise Boucher, confirmed the news and explained the change in a series of Instagram comments on Sunday.
After a curious fan asked if she renamed her son, the 32-year-old responded with the new spelling with the number 12 changed into the Roman numeral version, and added, "Roman numerals. Looks better TBH."
California law only mentions letters from the alphabet being used but Grimes said that "one dash is allowed."
Although the spelling of their baby's name has changed, the pronunciation remains the same. During his time as a guest on Joe Rogan's podcast, The Joe Rogan Experience, Musk explained how to say his son's name.
He explained that the X is pronounced like the letter and "the AE is like pronounced 'ash,'" then A-12.