Mark Twain three of his siblings survived childhood: Orion (1825–1897), Henry (1838–1858), and Pamela (1827–1904). His sister Margaret (1830–1839) died when Twain was three, and his brother Benjamin (1832–1842) died three years later. His brother Pleasant Hannibal (1828) died at three weeks of age. When he was four, Twain's family moved to Hannibal, Missouri, a port town on the Mississippi River that inspired the fictional town of St. Petersburg in "The Adventures of Tom Sawyer" and the "Adventures of Huckleberry Finn". Slavery was legal in Missouri at the time, and it became a theme in these writings. His father was an attorney
Mark Twain 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." 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 deep and it is safe to pass." Twain said that his famous pen name was not entirely his invention. In "Life on the Mississippi", he wrote: Captain Isaiah Sellers was not of literary turn or capacity, but he used to jot