How is Polly is short for Mary?!
Dick = Richard
Going back to the Middle Ages, there weren’t that many first names yet. And so many people were named Richard that everyone needed nicknames to tell one another apart. Richard was shortened to Rick, and then people would rhyme it with something else to become an entirely new name — so Rick became Dick. (And then the modern trend of being dirty little pervs meant that everyone today giggles when they hear of a guy named Dick.)
Bill = William
William was also a popular name in the Middle Ages, so many nicknames were born. It was shortened to Will, which turned into Bill. Rhyming nicknames strike again!
Nancy = Ann
Why is the short form of Ann (or Anne) actually longer than the original name? People used to use the affectionate phrase “mine Ann,” which eventually turned into “my Nan.” Nickname trends of the time also had people adding “-cy” to the ends of name, which is how Ann evolved into Nancy.
Fun fact: This means that sisters Ann and Nancy Wilson, members of the band Heart, were kind of named the same thing.
Ted and Ned = Edward
Yep, you’ve got it: “Mine Ed” turned into “my Ned.”
As for Ted, just as Richard and William were popular names, Edward was a very common name that required nicknames to be created. With names that start with vowels, people often added an easy-to-pronounce consonant, so Ed became Ted.