On this day in 1863, who used a pseudonym for the first time?

Samuel Langhorne Clemens (better known as Mark Twain) used that name as a pen name for the first time.

According to Mark Twain: Banned, Challenged, and Censored, the phrase mark twain is a riverboat term for two fathoms, or twelve feet. It was used to denote the line between safe and dangerous waters.

In this book, written by Michelle M. Houle, she discusses Twain's life and times, analyzes two of his best-known books, and explores both sides of the argument over censorship. She helps readers decide for themselves about whether Twain's books should be banned.