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Grimes revealed the update — changing the name from "X Æ A-12" to "X Æ A-Xii" — on her Instagram on Sunday.
"Roman numerals. Looks better [to be honest]," Grimes wrote in the comment section of her most recent Instagram post. "[O]ne dash is allowed."
Grimes explained the baby's unique name earlier in May. X represents "the unknown variable," Æ the "elven spelling of Ai (love [and]/or Artificial intelligence)" and A-12 the precursor to her and Musk's favorite aircraft the SR-71.
"No weapons, no defenses, just speed. Great in battle, but non-violent," Grimes wrote on Twitter, adding that A also stood for "Archangel," her favorite song.
Musk and Grimes welcomed the baby, her first child, on May 4.
Musk has five sons from his previous marriage to author Justine Wilson.