Jack was C.S. Lewis’s childhood nickname and the name by which he was commonly known to family and friends.
Lewis was born Clive Staples Lewis, but after the death of a childhood dog named Jacksie, he began using “Jacksie” and later shortened it to “Jack.” The nickname remained with him for the rest of his life. Many letters, memoirs, and biographies therefore refer to him as Jack Lewis rather than Clive Lewis.
The initials C.S. came from his formal name, Clive Staples. “Staples” was his mother’s maiden name, and Lewis used the initials professionally as an author and scholar. The nickname does not represent a different pen name or separate literary identity.
Warren Lewis, Jack’s older brother, also wrote memoirs about their family and life together. Their close relationship influenced the preservation of many details about Lewis’s childhood, military service, Oxford career, and later years.