Livre d'Or

As I understand it, though, common law only lets you have one real name.

So if your birth certificate says Charles but everyone calls you Tiger, then Tiger can be your real name for all intents and purposes. But then you can't use Charles as your name any more.

Depends how you do it: yes, there is the "renounce and relinquish" language, but I changed my name by deedpoll recently. I went from X Y [mother's surname] [father's surname] to X Y Z [mother's surname].

In terms of everyday usage I'm Z [mother's surname], where previously I was X [father's surname]. But I still have the right to use the names X and Y...