1 Answer

Under the Constitution, all Commonwealths are also States, and as you know, all States in the U.S. have similar forms of government and supposedly equal powers. Four States refer to themselves as Commonwealths: Massachusetts, Pennsylvania, Virginia, and Kentucky (Kentucky was part of Virginia until it became the 15th State). These four are Commonwealths mainly because the term is used in their Constitutions. The founding fathers were steeped in the political theories of Locke, Hobbes, and other English philosophers of the 1600s. So when men like John Adams in Massachusetts and George Mason in Virginia wrote their states’ constitutions in the 1770s, ’80s and ’90s (a politically sensitive time!), they used the word “commonwealth” to make a very clear point of the fact that their governments were based on the authority of the people, i.e. the entire body politic, and not The Crown. So they are Commonwealths simply because they elected to call themselves Commonwealths.

However, while most of the resources I found say there is no difference, WikiAnswers.com offered one slight difference, “in a commonwealth landowners do not have mineral or oil rights to their land. They don’t actually own their land but own its use. However this does not much differentiate their structure or self-government in any way from other states in the Union.”

The venerable Dictionary.com offers an alternate definition: a “self-governing, autonomous political unit voluntarily associated with the United States, namely, Puerto Rico and the Northern Mariana Islands.” The degree of dependency varies from place to place, but we’ll use Puerto Rico as an example.

http://deep.mastersfamily.org/2009-06/the-difference-between-a-state-and-a-commonwealth/