What was C.S. Lewis’s birth name before he became known as “Jack”?

C.S. Lewis’s birth name was Clive Staples Lewis.

Lewis was born in Belfast on 29 November 1898, and the initials in his pen name come from Clive Staples. As a boy, however, he began calling himself “Jacksie” after the death of his dog, Jacksie. The nickname was later shortened to “Jack,” and friends and family generally used it throughout his life.

Readers sometimes assume that C.S. stands for a more formal literary pseudonym, but it was simply Lewis’s own initials. He published fiction, literary criticism, apologetics, and devotional works under the name C.S. Lewis, while “Jack” remained his familiar personal name.

The name distinction also appears in biographies and letters. Warren Lewis, his brother, used “Jack” when referring to him, as did many members of the Inklings. Lewis’s professional identity and personal nickname therefore coexisted without being separate identities.