Twain used different pen names before deciding on "'Mark Twain". He signed humorous and imaginative sketches as "Josh" until 1863. Additionally, he used the pen name "Thomas Jefferson Snodgrass" for a series of humorous letters.[110]
He maintained that his primary pen name came from his years working on Mississippi riverboats, where two fathoms, a depth indicating safe water for passage of boat, was measured on the sounding line. Twain is an archaic term for "two", as in "The veil of the temple was rent in twain." [111] The riverboatman's cry was "mark twain" or, more fully, "by the mark twain", meaning "according to the mark [on the line], [the depth is] two [fathoms]," that is, "The water is 12 feet (3.7 m) deep and it is safe to pass."