He maintained that his primary pen name came from his years working on Mississippi riverboats, where two fathoms, a depth indicating water safe for the passage of boat, was a measure on the sounding line. Twain is an archaic term for "two", as in "The veil of the temple was rent in twain."[146] 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."