Why a people with the name william called Bill?

Some of Best Answers which i got it from net :

1)
Because the ancient Greek philosophers' symbol for the letter "W' was some sort of alteration of the letter "B". So, instead of Will for short, people started to write Bill. And now in today's modern age that is the most common name for somebody named William


2)
William is sometimes shortened to Will too. It is because it was shortened to Will originally, that it became 'Bill':

Will(iam) > Bill was, in fact, part of a great 13th-14th century trend of swapping some other letter for the original first letter of a name as rhyming slang. There were hundreds at the time but most have now died out. Other names resulting from this process include: Polly from Molly, Bob from Rob (from Robert), Hick and Dick (from Richard), Hodge from Roger, Molly from Mary, Sally or Sadie from Sarah, Dolly from Dorothy, Harry from Harold, Betty or Bess from Beth (from Elizabeth).

Many of the original names from which these nicknames were formed were Norman French names. William is the anglisied version of Guillumme, an old French name. It appears that the changes were part of the English adapting them to their own language. Apparently one impetus (there were others) in the trend for letter swapping at that particular time was a dislike amongst the native English for the harsh Norman French "r".
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