Why is William, Billy?

[muckandtwigs]

Being a William and having always been addressed as Billy, a lady asked me why people called William are called Billy? How could I be expected to know, it's only a recent thing with me,  (the last 70 years).
 

[conahy calling]

http://mentalfloss.com/article/24761/origins-10-nicknames

Link gives some info on your query.

The other names on the link are

Dick     =       Richard
Hank             Henry
Jack              John
Chuck           Charles
Peggy           Margaret
Ted               Edward
Harry            Henry
Jim               James
Sally             Sarah     

[pinefamily]

And then it only gets really confusing when Ted can also be short for Theodore. Don't get me started with Bert....... 

[muckandtwigs]

I can see the Ted in Theodore and the Bert in Albert, but where is the 'B' in William.

[KGarrad]

I think it's partly rhyming tradition (e.g. Dick for Rick?), and partly because hard consonants are easier to pronounce?

So, Richard became Rich, then Rick, then Dick.
William became Willy or Will, and then Billy or Bill.

[LizzieL]

Why Peggy for Margaret?

[pinefamily]

Bert was/is Albert, Wilbert, Norbert, Bertram, but why not Robert?

[loobylooayr]

Bert was/is Albert, Wilbert, Norbert, Bertram, but why not Robert?

Bert/ Bertie can be short for Robert in Scotland    (Trust us ...)
I'd an Uncle Bertie whose "proper name " was Robert and I know of 2 other Berts who started out as Roberts.

Looby

[LizzieL]

I had a great uncle Bertie, which I always thought was short for something, but later found his birth was registered as Bertie.

On other side of family had a great aunt Daisy, who I had great trouble finding marriage record for, until I was told by another relative that her real name was Marguerite - which as gardeners will know is a posh daisy