What is the difference between the first and second continental congress?

The First Continental Congress was far shorter in duration. It officially convened on September 5, 1774, and adjourned on October 26 of that year - just 51 days, or just over seven weeks later. When it adjourned, it stipulated that it should reconvene on May 10, 1775, if the underlying contentious issues with Britain hadn't yet been resolved. They weren't, and so it reconvened as the Second Continental Congress.

The Second Continental Congress, in contrast, was the de facto American government of the fledgling United States of America, during the entire American Revolution, and beyond. It lasted for several years.
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