Just something I'm thinking about in terms of addressing invites... I always thought it would be less formal to put someone's nickname on an invitation but now I'm wondering if it is weird to address an invite using someone's full name if they never ever use it.
For example, I have a friend named Katie, who's full name is Katherine. She has literally never been called Katherine by me or anyone else in her life. Same for a friend named Sam (real name is Samuel but again, no one has ever called him that).
Is it weird to use someone's full name if they literally never use it themselves? Is it too informal to use someone's nickname, if that is the name they exclusively go by? What did you all do?