What was C.S. Lewis’s full first name, usually shortened to “C.S.”?

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

He was born Clive Staples Lewis in Belfast, Ireland, on 29 November 1898. “C.S.” became the form by which he was publicly known as an author, scholar, broadcaster, and Christian apologist.

Lewis reportedly disliked the name Clive and used “Jack” among family and friends from childhood. That nickname is why biographies often introduce him as “Jack Lewis,” even though his published name was C.S. Lewis.

The initials can cause confusion because readers sometimes expand them as Charles or Christopher. Neither is correct: the initials stand for Clive Staples. His brother, Warren Hamilton Lewis, was commonly called Warnie.