What was C.S. Lewis's full first name, rather than the initials he used publicly?

C.S. Lewis's full first name was Clive Staples. He was born Clive Staples Lewis in Belfast, Ireland, on 29 November 1898, and published almost all of his major works under the initials C.S. Lewis.

Lewis disliked the name Clive during childhood and was known to friends and family as Jack. The nickname came from his childhood association with a dog called Jacksie, whose death led him to adopt Jack as his preferred name. That nickname remained with him throughout his adult life.

The initials helped create the familiar author identity attached to books such as The Chronicles of Narnia, Mere Christianity, and The Screwtape Letters. His middle name, Staples, came from his mother’s family. Lewis’s brother Warren, usually called Warnie, was a lifelong companion and later edited several of Lewis’s posthumously published works.