Clive Staples Lewis was often called "Jack".

Lewis Carroll's real name was Charles Lutwidge Dodgson. As he was a deacon at Christ Church College, Oxford University, he is given the title Reverend, so his name is often given as Rev. Charles Lutwidge Dodgson. He wrote books about mathematics using his real name.

The initials stood for 'Clive Staples' but throughout his life Lewis was referred to by family and friends as 'Jack'. The reason for this was that when he was just four years old, the family dog (named Jacksie) was run over by a car. Young Clive decided then he would only be called Jacksie - and it was only as he grew older that his mother was able to cionvince him to at least change it to Jack.

C. S. Lewis (Clive Staples)

Clive Staples Lewis commonly referred to as C. S. Lewis and known to his family and friends as Jack (: