This closed figure, with straight sides, is known as a Polygon.
A polygon.
In the UK it is a legal right to join a union, so they can't legally stop you. http://www.direct.gov.uk/en/Employment/Employees/TradeUnions/DG_10027560 Read up on the law if you are in the US, a good start would be here... http://law.enotes.com/everyday-law-encyclopedia/teacher-s-unions-collective-bargaining Also, contact the union that you would like to jointo make sure of your facts, get them to tell you of your legal rights to join and if its legal- quote law at them and do it anyway.