What were C.S. Lewis's first two given names?

C.S. Lewis's first two given names were Clive Staples.

His full birth name was Clive Staples Lewis. The initials C.S. became the form used on his books, essays, and academic work. In private life, however, he was widely known as Jack, a nickname that originated when he was a child and chose to identify with a dog named Jacksie.

Lewis was born into an educated family in Belfast. His father, Albert James Lewis, was a solicitor, and his mother, Florence Augusta Hamilton Lewis, was the daughter of a Church of Ireland priest. His older brother Warren was usually called Warnie.

A common mistake is to expand “C.S.” into a different pair of names. The initials stand for Clive Staples, not Charles or Christopher. The name Jack is also not a formal middle name; it is Lewis's familiar nickname.