What was C.S. Lewis’s birth name before he adopted the name Jack?

C.S. Lewis’s birth name was Clive Staples Lewis. He was given that name at birth in Belfast on 29 November 1898, and the initials C.S. Lewis became the name under which he published his major works.

The name “Jack” came from a childhood event involving his dog, Jacksie, which was killed by a motorcar. Lewis later rejected his given first name and asked family and friends to call him Jacksie, soon shortened to Jack. The nickname remained with him for the rest of his life.

Readers often assume that “C.S.” stood for a longer pen name or that Jack was a separate formal name. It was not: the initials represented Clive Staples, while Jack was his preferred personal name.

His middle name, Staples, came from his mother’s family. His published identity and private nickname therefore developed in different ways.