C.S. Lewis commonly used the nickname Jack instead of Clive. His full name was Clive Staples Lewis, but he adopted Jack as a personal name during childhood.
According to accounts of his early life, the change followed the death of a childhood dog named Jacksie. Lewis reportedly began referring to himself as Jacksie and later shortened the name to Jack. The nickname stayed with him throughout adulthood.
Family members, friends, and colleagues generally called him Jack rather than Clive. This is why biographies, letters, and recollections often describe “Jack Lewis,” even though his published books appeared under the initials C.S. Lewis.
The initials concealed his first and middle names in a professional literary style. They also helped create a distinction between the public author and the informal friend known to people in his Oxford circle, including J.R.R. Tolkien and Charles Williams.