C.S. Lewis commonly used the nickname Jack.
Lewis was born Clive Staples Lewis, but he adopted the name Jack during childhood. According to accounts of his early life, the change followed the death of his childhood dog, Jacksie. The boy began referring to himself by that name, which gradually became “Jack.”
Friends, family members, and colleagues generally called Lewis Jack rather than Clive. The nickname remained part of his personal identity throughout his adult life, even while his books appeared under the initials C.S. Lewis.
The initials came from his full name: Clive Staples Lewis. “Staples” was a family name, not the nickname, and “Jack” was not a shortened form of “Clive.” These distinctions explain why biographical writing often refers to him as Jack Lewis.