What does the nautical term 'mark twain' signify?

The nautical term “mark twain” signifies two fathoms deep.

A fathom is a traditional nautical unit of depth equal to six feet, so two fathoms means 12 feet, or about 3.66 meters. On a riverboat, a leadsman measured the water with a weighted line marked at set intervals. When the sounding indicated two fathoms, the call “by the mark twain” warned the pilot that the vessel had reached a particular depth.

The word “twain” is an old English term meaning “two,” as in the phrase “cleave in twain.” On the shifting Mississippi River, a depth of two fathoms was often considered enough water for a steamboat to pass safely, although the required depth depended on the vessel and local conditions.

Samuel Clemens adopted “Mark Twain” as his pen name after working as a Mississippi River pilot. The name is often treated as his invention, but Clemens said he borrowed it from Captain Isaiah Sellers, who used it in river reports. It does not mean one, three, or four fathoms.