What is the full name behind the initials C.S. Lewis?

The full name behind C.S. Lewis is Clive Staples Lewis.

He was born Clive Staples Lewis in Belfast in 1898. “Clive” was his given name, while “Staples” was a family name connected to his mother’s side. As a child, he adopted the nickname “Jack,” a name used by family and friends for the rest of his life. The initials C.S. became the public identity under which he published fiction, scholarship, and Christian apologetics.

The name “Jack” can cause confusion because it was not a pen name replacing C.S. Lewis. It was Lewis’s preferred personal nickname. He also wrote some works under different publication arrangements, but his major books—including The Screwtape Letters, The Great Divorce, and the Narnia novels—are associated with C.S. Lewis. His brother Warren was usually called Warnie.