Bing implies there are Only 7 Founding Fathers (of the USA); How did 'The Seven' Get onto That List?

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Oh, there were many other 'Founding Fathers' supporting The Seven; but Wikipedia confirms that The Seven (on Bing's "search-results page") were the ones originally identified as such (by historian Richard B. Morris identified seven figures as key Founding Fathers in his book Seven Who Shaped Our Destiny: The Founding Fathers as Revolutionaries). How did they get on 'the list'? I long thought it was basically 'all the guys who signed The Declaration of Independence (of the USA).' But no ... I mean, that's one trait that a lot of the Founders (both The Seven and the "Other Patriots"); but the title 'Founding Father' generally goes to ANYONE who played a key role in establishing the United States of America!