S.J.D., of Belleville posed this question to The Answer Man:
Q. Why do we have two very different names for a couch/sofa? When you look on craigslist, they are used interchangeably. Is there a difference?
Interesting article about the history of the name!
I have some further input on the question. My husband used to call the sofa a couch or davano until I corrected him Now he's pretty good at just using sofa for the multiple seat thing in the den or living room, usually taken by the dog.