Thread by @swardley

X : Your son is Henry.
Me : Yep.
X : Why do you call him Harry?
Me : Harry is short for Henry.
X : Prince Harry?
Me : Henry Charles Albert David
X : What?
Me : Bob is short for Robert. Bill for William. Liz for Elizabeth, Sue for Susan, Dick for Richard. Harry for Henry ...

... these are all short form names, alternative ways of saying the given name. Why?
X : My son is called Harry.
Me : Cool. Do you mean Henry?
X : No, I mean Harry.
Me : Ok, well that's cool.
X : So, I think your son should stop calling himself Harry.
Me : Well, you can sod off.

X : Well, it's not right.
Me : Which bit? You name your child after the short name, the nickname and expect everyone else to now follow your rules? No, I don't think so. Henry can call himself Harry as much as he wants.

X : But surely it should be Henry?
Me : My grandfather was Harry, my son is named after him. Both given names are Henry ... Harry has been the pet name for Henry for centuries. You don't think Prince Harry means the given name is Harry do you?
X : Yes
Me : Hmmm.

Me : Do you go upto people and say "Why do you call yourself Bob, your name is Robert." or "Why do you call yourself Bill, your name is William"?
X : It's not the same.
Me : Well ... it is. Exactly the same. I wouldn't. Centuries of tradition and no-one made you name police.