What was C.S. Lewis’s birth name, before he adopted the name used in his books?

C.S. Lewis’s birth name was Clive Staples Lewis. He was born with the first name Clive, but as a child he adopted the nickname “Jack,” which became the name used by family and friends.

Lewis’s initials came from his given names: Clive Staples Lewis. “Staples” was his mother’s maiden surname, and Lewis used the initials C.S. professionally as an author and scholar. The nickname Jack has a separate origin in his childhood after the death of a much-loved dog named Jacksie; he began calling himself Jacksie and later shortened it to Jack.

Because his books were published as C.S. Lewis, many readers know the initials better than the full name. Jack is not his birth name, and it was not a pen name in the usual sense. It was the personal name by which he was widely known.