The “C” in C.S. Lewis stands for Clive.
Lewis’s full birth name was Clive Staples Lewis. He was born in Belfast in 1898, and “Clive” was the name recorded for him even though he later became universally known by his initials. As a child, he reportedly adopted the nickname “Jack,” a name used by his friends and family for the rest of his life.
The initials C.S. became the author’s public identity on books about Christianity, literature, mythology, and imaginative fiction. They also helped distinguish his authorial persona from the private nickname Jack. His middle name, Staples, came from his mother’s family.
Readers sometimes guess Charles because C.S. is a familiar set of initials among writers, but Lewis was not named Charles. His unusual combination of formal name, literary initials, and lifelong nickname is one reason biographies often explain all three names together.