One of the most famous rappers in the world performs under a name that is just two letters, said out loud.
He was born Marshall Bruce Mathers III on October 17, 1972, in St. Joseph, Missouri. As a child he moved often with his mother, between Missouri and Michigan, before they settled in Detroit when he was 12.
At 14 he started rapping with a high-school friend, Mike Ruby. According to Wikipedia, the two took the names "Manix" and "M&M." The second was his initials, Marshall Mathers. Over time "M&M" became "Eminem," the same sounds written out as a word.
The spelled-out form is the one that stuck. Why he made the change is less well documented than the change itself. A widely repeated explanation is that "M&M" was too close to the chocolate candy of the same name. Popular articles repeat this, but we found no statement from Mathers or his label confirming it, so it is best treated as a likely guess rather than a known fact.
The initials were not his only early name. When he formed his first group, the New Jacks, in 1988, he went by MC Double M. That is another way of saying the same two letters.
Later he added a second persona. Slim Shady was the violent, sadistic alter ego that first drew wide attention, starting with the Slim Shady EP of 1997. That record reached Interscope Records after he competed in the 1997 Rap Olympics in Los Angeles.
His major albums then used each of his three names in turn. The Slim Shady LP came out in February 1999. The Marshall Mathers LP followed in May 2000, and The Eminem Show in May 2002. Each title leaned on a different side of the same person: the character, the man, and the performer.
Turning letters into a word is an old habit in English. The word emcee, for a master of ceremonies, was first spelled "emsee" in 1933. It is simply the letters M.C. written as they sound, which is exactly what happened to M&M.