C.S. Lewis’s first name was Clive.
He was born Clive Staples Lewis in Belfast, Ireland, on 29 November 1898. “C.S.” uses his first and middle initials, while “Jack” was the familiar name he adopted and used throughout most of his life. Friends and family generally called him Jack rather than Clive.
The name “Jack” began in childhood, after the death of his dog Jacksie. Lewis reportedly rejected his birth name for everyday use and gradually became known by the shorter nickname. This can cause confusion because biographies may introduce him as Clive Staples Lewis but discuss him almost exclusively as Jack.
Lewis later became famous as a literary scholar, Christian writer, broadcaster, and author of The Chronicles of Narnia. His initials are therefore better known internationally than either of his given names.