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

C.S. Lewis’s birth name was Clive Staples Lewis. He was born with that full name in Belfast in 1898, and the initials C.S. became the form by which readers and scholars usually identified him.

Lewis disliked the name Clive from childhood and preferred the nickname “Jack,” a name he adopted after a childhood dog called Jacksie was killed. Friends and family continued to call him Jack for the rest of his life, even though his books appeared under C.S. Lewis.

The initials can cause confusion because some readers assume the C stands for Charles or Christian. It stands for Clive, while S stands for Staples, his mother’s family name. “Jack” was his personal name, not a pen name used on his publications.