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Grimes and her billionaire lover Elon Musk have altered the name of their newborn son in order to comply with California law.

Their child, who arrived earlier this month, was initially called X Æ A-12 (pronounced 'Ex Ash A Twelve'), but the law in his birth state does not allow numbers to be used in official names.

Answering fans' queries on her Instagram account, the 32-year-old singer revealed that the baby's official name is now written X Æ A-Xii, with the Roman numerals signifying the number 12 used instead.

An Instagram commenter asked: "Did you change the baby name because of Californian laws? What is the baby's new name?"

Grimes responded by writing: "X Æ A-Xii," prompting a different Instagram user to write: "Nice! Just removed the numbers to confirm to California law."

The Genesis hitmaker added, "Roman numerals. Looks better (to be honest)," then continued, "one dash is allowed."

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.

The couple have explained the unusual name to fans by revealing that X stands for the "unknown variable", Æ is her "elven spelling of AI", and the A-12 is the forerunner to the SR-71 plane - their favourite aircraft.