The regional governing body of the Presbyterian Church (U.S.A.) is the presbytery. A presbytery consists of the ministers in its area plus an equal number of elder commissioners elected by the sessions of its churches.
There are currently 172 presbyteries in the United States. A presbytery is responsible for the mission and government of the Church throughout its geographical region. This consists of many things, including working with churches to call pastors, developing regional strategies for mission and ministry, and coordinating the ministries of its churches. It is interesting to note that commissioners to presbyteries and other governing bodies are not bound to vote as they believe those who elected them would want them to vote. Rather, they are bound to vote their consciences. This acknowledges that the Holy Spirit works among us and frees our commissioners to respond to the working of the Spirit through them.
The General Assembly is the national governing body of the Presbyterian Church (U.S.A.). The General Assembly meets annually to consider and legislate matters of national significance to the Church. Presbyteries elect an equal number of minister and elder commissioners to represent them at each General Assembly. One of the primary responsibilities of the General Assembly is to consider and vote on amendments to the Church's constitution.
The constitution of the Presbyterian Church (U.S.A) is made up of 2 volumes, the Book of Confessions and the Book of Order. The first of these contains confessional statements of the Church from throughout the ages. The earliest confession in the book is "The Apostles' Creed" which has its roots in the second century. The latest is "A Brief Statement of Faith" which was written just a few years ago. The confessions guide the Church in its faith and practice, although they are subordinate to the Bible. The Book of Order defines the Church's form of government, contains directions and recommendations for worship, and provides rules for dealing with people or governing bodies who choose to break the 'laws' of the Church.
There is a fourth level of governing bodies in the Presbyterian Church (U.S.A.) that stands alongside the other three; namely, the synods. The United States is divided into 15 synods, each of which is made up of the presbyteries in its area. The synods stand alongside the other three levels of governing bodies in that presbyteries elect commissioners to the synods, but the synods do not have any direct representation in the General Assembly. The synods are responsible for various administrative functions and have jurisdiction over the churches in their areas.
It is no coincidence, by the way, that there are several similarities between the Presbyterian form of government and that of the United States. Several Presbyterians were instrumental in the formation of the United States government. John Witherspoon, for example, was a Presbyterian minister.