The Episcopal Diocese of Pittsburgh's diocesan convention voted yesterday to secede from the Episcopal Church. Many conservative Episcopalians take the view (erroneous, in my judgment) that any Episcopal diocese can secede According to these folks, a diocese voluntarily 'joins' TEC and can voluntarily withdraw at will.
This view, however, seems to presuppose that a given diocese was formerly an independent, free-standing association or entity that existed outside TEC, and which mutually agreed with TEC to change that status. Think of the Republic of Texas joining the Union in 1845 following nine years as an independent state. (Or as was half-seriously proposed a number of years ago, Canada's Western provinces deciding to become the 51st etc. states of the United States.)
A handful of Episcopal dioceses may have originated in this way. My guess, however, is that for most dioceses the reality is otherwise. Many Episcopal dioceses originated, not as free-standing churches like the Republic of Texas, but because Episcopalians 'colonized' mission territory as an extension of the existing Episcopal Church, eventually petitioning for full diocesan status in the church. This would be comparable to, say, the states of Oklahoma, Arizona, and Alaska: If memory serves, each of those states was colonized by Americans as an extension of the United States (I'm not going to get into the rights-of-indigenous-peoples argument here), and later sought and received full statehood status from the Congress.
Ironically, two of the present cases involve the Episcopal dioceses of San Jose and, in all likelihood, Fort Worth. Each began when it was carved out of an existing diocese by authority of the General Convention. In that regard, these dioceses are like the state of Maine, which was carved out of the existing state of Massachusetts by Congress as part of the Missouri Compromise of 1820.
In the coming months, it will be useful to keep in mind these distinctions of origin, I think.