Ah, the Twain Name Game. It is, indeed, an intriguing topic, for our man seems to have been known by different names to different people: Sam or Sammy to Hannibal friends, to his mother and brother, and to mining chums; usually Clemens to Howells and Rogers (as they were Howells and Rogers to him, in the Victorian tradition of gentlemen friends using last names -- think Holmes and Watson); Mark to Twichell and other close friends; Youth to Livy. And he signs his letters differently, according to the recipient. I kicked this around a bit in "The Shape of the River" book, just for the sheer fun of it. It's just as intriguing (to me, at least) to see the primary name different biographers choose to employ. Some go with Sam. Some with Mark. Some go with Clemens. Some with Twain. Symbolically, I think it says a great deal about the richness of the writer and the personality we find so fascinating, but I'm not sure what. Maybe it's like the shape of the river, as Capt. Bixby explains it to young Sam (not yet Mark). Each time you try to get a fix on him, Mark-Sam-Clemens-Twain goes fooling around on you, changing form and daring you to keep up with him.