C.S. Lewis’s first name was Clive, although he was widely known by his initials rather than by that given name.
He was born Clive Staples Lewis in Belfast, Ireland, on 29 November 1898. The middle name “Staples” came from his mother’s family; it was not a title or a pen name. As a boy, Lewis adopted the nickname “Jack,” and friends and family generally used that name throughout his life.
The initials “C.S.” became strongly associated with his books on Christianity, literary criticism, and imaginative fiction. Readers sometimes assume that the “C” stands for “Charles,” partly because Charles was a common name among British writers of his era. It does not. Lewis’s better-known informal name, Jack, also does not appear in the initials. The name Clive therefore identifies the first part of his legal name, while Jack identifies the personal name he preferred in everyday life.