Fanny = Frances???

[LoneyBones]

Yep! Never take it for granted.
Fanny and Frances can be interchangeable but thay can also both be names in their own right.
This also goes for Francis and Frank, John and Jack, James and Jim....... the list is endless, Ann, Annie, Nan, Nancy....
Leonie.

added;
Just to confuse you,  there is also Myfanwy known as Fanny.

[forthefamily]

A very annoying name for me 

Frances....Fanny....Frannie....Fran 

Francis....Frank....Franky....Frankie 

Elizabeth is my next least favorite name in the census records 

mab

[mare]

Sorry ... your question has already been well answered, that it is commonly used as a form of Frances but also just as a popular name of the time ... as Johngirl has alluded to, not a good choice now. In addition to the alternative Leonie gave, Fanny is a derivative of Euphemia and Stephanie..

  mare