A day after Tesla CEO Elon Musk said his baby boy will be called 'X Æ A-12 Musk', his girlfriend Grimes tweeted to explain the meaning of their newborn's name. 'X' refers to "the unknown variable", 'Æ' is "my elven spelling of Ai (love &/or Artificial intelligence)," she said. "A-12 = precursor to SR-17 (SR-71) (our favourite aircraft)," she added.