What does Mark Twain mean and where did Samuel Langhorne Clemens come up with this pseudonym? “On river boats, one member of the crew always stood near the railing measuring the depth of water with a long cord which had flags spaced a fathom (six feet) apart. When the crewmen saw the flags disappear he would call out ‘Mark One!’ for one fathom and for two fathoms he called out ‘Mark Twain!’ Two fathoms meant safe clearance for river boats, so Samuel Clemens chose a name which not only recalled his life on the river, but which als ....