Elon Musk and his partner, singer Grimes, have changed the name of their son.

Grimes, real name Claire Boucher, gave birth on May 4 and announced the name as X Æ A-12 Musk. However, Californian law states a baby’s name can only have the letters from the alphabet in it.

After she posted a new photo on Instagram, a fan asked Grimes: “Did you change the baby name because of Californian laws? What is the baby’s new name?” 

She responded, saying the baby’s name is now ’X Æ A-Xii’.

When other fans continued to ask her why she and Musk changed the name, she did not mention the law and just said: “Roman numerals. Looks better tbh”.

A Californian lawyer confirmed to People magazine that the couple would not be able to proceed in registering their child’s name with numbers in.

David Glass said: “In California, you can only use the '26 characters' of the English language in your baby name.”

At the time of the original baby name announcement, Grimes explained the meaning of it to fans who were confused.

She said: "X, the unknown variable. Æ, my elven spelling of Ai (love &/or Artificial intelligence).

"A-12 = precursor to SR-17 (our favorite aircraft). No weapons, no defenses, just speed. Great in battle, but non-violent."