A voluntary group or union (also sometimes called a voluntary organization, common-interest association, association, or society) is a group of individuals who enter into an agreement, usually as volunteers, to form a body (or organization) to accomplish a purpose. Common examples include trade associations, trade unions, learned societies, professional associations, and environmental groups. Membership is not necessarily voluntary: in order for particular associations to function correctly they might need to be mandatory or at least strongly encouraged, as is common with many teachers unions in the US. Because of this, some people use the term common-interest association to describe groups which form out of a common interest, although this term is not widely used or understood. Voluntary associations may be incorporated or unincorporated; for example, in the US, unions gained additional powers by incorporating. In the UK, the terms Voluntary Association or Voluntary Organisation cover every type of group from a small local Residents' Association to large Associations (often Registered Charities) with multimillion-pound turnover that run large-scale business operations (often providing some kind of public service as subcontractors to government departments or local authorities).