C.S. Lewis commonly used the first name Jack instead of Clive.
Lewis was born Clive Staples Lewis, but he adopted the name Jack after a childhood event involving his dog, Jacksie. Following the dog’s death, the young Lewis reportedly insisted on being called Jacksie, a name that later developed into Jack. Family members and close friends used Jack for the rest of his life.
The name explains why biographies and personal accounts often refer to him as Jack Lewis rather than Clive Lewis. His published works, however, generally appeared under the initials C.S. Lewis, a professional name that became internationally recognized.
Jack was also the name used by members of the Inklings and by other close associates. It should not be confused with his middle name, Staples, which came from his mother’s family. The initials C.S. stand for Clive Staples, not for the name Jack.