An ex officio member is a member of a body (a board, committee, council, etc.) who is part of it by virtue of holding another office. The term is Latin, meaning literally "from the office", and the sense intended is "by right of office"; its use dates back to the Roman Republic.
A common misconception is that the participatory rights of ex officio members are limited by their status. This is incorrect, although their rights may be indeed limited by the by-laws of a particular body. Robert's Rules of Order, Newly Revised (10th ed.), clarifies that the term denotes only how one becomes a member of a group, not what one's rights are. It is a method of sitting on a committee, not a class of membership (466-67). Frequently, ex officio members will abstain from voting, but unless by-laws constrain their rights, they are afforded the same rights as other members, including debate, making formal motions, and voting (466-67; 480).
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