What was C.S. Lewis's first name, commonly shortened to “C.S.”?

C.S. Lewis's first and middle names were Clive Staples.

He was born Clive Staples Lewis in Belfast, Ireland, on 29 November 1898. His family used the nickname “Jack,” which he adopted during childhood and retained throughout his life. The initials “C.S.” came from his given names, not from a pen name created for his books.

Lewis is often identified simply as C.S. Lewis because that form appeared on his major works, including The Chronicles of Narnia and Mere Christianity. A common mix-up is assuming that “Jack” was a shortened form of Clive; it was instead a personal nickname. His brother Warren was also known as Warnie, another family nickname that appears frequently in biographies of Lewis.