Why is William, Billy?

[KGarrad]

Why Peggy for Margaret?

Same reasons as I said before?!

Margaret can be shortened to Meg, Maggie or Meggie.
Changing to a hard consonant gives us Peg and Peggy?

[Jebber]

How about Polly for Mary?

[loobylooayr]

How about Polly for Mary?

Mary----Molly----Polly (I think??? )
 

[pinefamily]

I have also seen Molly for Matilda. As well as Tilly.

[cavvytri]

yes I had an Aunt Polly who was a Mary, an Aunt Daisy who was a Marguerite and an Aunt Peggy who was a Margaret!

[Josephine]

And people named Sarah who are called Sadie.

[Drosybont]

The one which baffled me for quite a while was a Martha Jane known as Pat in the family until she was in her late teens.  1850s to 1860s, family living in Cardiff but came from Pembrokeshire.  Mother switched between the two in her diary, thought at first there were two daughters, finally sorted it when the girl fell on the way to market, noted under one name, and was laid up in bed with a bad leg the next day, under the other name!  Pat seems to have been a commonly used family name for Martha Jane in Pembrokeshire, not sure about elsewhere.  No idea why.

Drosybont

[Marmalady]

Martha --- Mattie ---- Pattie

My husband had a Great Aunt Pattie who was really a Martha  (Derbyshire)


But a real puzzler is Bunty for Margaret -- any ideas?

[KGarrad]

I don't think Bunty is short for anything?
It appears to be a dialect word that means plump?!

http://www.britishbabynames.com/blog/2012/09/bunty.html