In some cases it's quite clear - a friend of mine is universally known by his middle name, primarily because it was going to be his first name until his parents realised that they were about to saddle him with the initials MAD...

In other cases, it's not so clear. A great-uncle was registered with given names of "Charles Edward". He was universally known as Ted - but does that mean his Given Name Used was Edward? Or was Ted his Nickname? How could you ever tell?
Then quite recently I was in touch with a distant relative whose grandfather had the registered first names of William Ernest. Slightly warily I wrote something along the lines of "Uncle Ern (as my mother knew him)" - and yes, his granddaughter was astonished that anyone called him that - even his wife called him Bill, she said!
And what triggered this was my discovery in a funeral report of a reference to the above Uncle Ted sending a wreath from "Ted and Vicky". The reason my jaw dropped was that Uncle Ted's wife was always known to us as Auntie Gertie - I had to double check in the database to confirm that her middle name was indeed Victoria. I suspect there was a reason for this - both Ted and his brother Stan married wives named Gertrude, so we always followed the rubric of naming them as "Uncle Ted's Auntie Gertie" or "Uncle Stan's...". I'm guessing that among Ted's siblings, his wife opted for "Vicky" to save confusion... But I'll never know
