Grimes and Elon Musk's legal name for their son has been confirmed in a birth certificate filing.
The couple had planned to call the little boy X Æ A-12 (pronounced 'Ex Ash A Twelve'), but the law in his birth state of California does not allow numbers to be used in official names.
Answering fans' queries on her Instagram account, the 32-year-old singer recently revealed that the baby's official name had been changed to X Æ A-Xii, with the Roman numerals signifying the number 12 used instead.
However, according to TMZ, Grimes and Musk had to make further alterations when they realised birth names must be limited to the English alphabet - no numbers or special signs - in the state.
According to California's rules on names, on birth certificates they must use "the 26 alphabetical letters of the English language" but can include some punctuation marks like apostrophes and dashes.
Finally, they settled on X AE A-XII Musk, with first name X, middle name AE A-XII, and Musk as his surname.
The couple previously explained the unusual name to fans by revealing that X stands for the "unknown variable", Æ (now AE) is her "elven spelling of AI", and the A-12 (A-XII) is the forerunner to the SR-71 plane - their favourite aircraft.
Grimes, real name Claire Boucher, and Musk's baby was born last month.