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Elon Musk and Grimes saved a surprise for son X Æ A-12's birth certificate

X marks the spot.

Tesla and SpaceX CEO Elon Musk and singer Grimes welcomed a son in May, and the baby's name has been a subject of interest ever since. Last month, Musk said on The Joe Rogan Experience podcast it was "X Æ A-12," pronounced "X Ash A Twelve." Now, an image of the birth certificate reveals only one letter of that is the baby's official first name. 

A photo of the document shared by TMZ shows the baby's first name is simply the single letter X, the middle name is AE A-XII, and the last name is Musk. 

"XII" is the number 12 expressed in Roman numerals, and seems like a way to get around the fact that California allows letters but not numbers on birth certificates.

Fans on Instagram asked Grimes what she calls the baby, and she responded that the child "has many names," according to CNET sister site

The baby was born on May 4. Musk said on The Joe Rogan Experience that this made him happy, since that date is commonly known as Star Wars Day for the saying "May the fourth (Force) be with you."