The board examines and interprets pertinent contractual provisions in the context of statutory requirements, its own past rulings, and those rendered by the U.S. Supreme Court. Arbitrators, on the other hand, apply the contract's written word, without considering the National Labor Relations Act or conducting an indepth analysis of board decisions. Union security clauses requiring dual unionism, accretion arrangements, and closed shop provisions are often enforced by arbitrators. The board considers such provisions to be improper, however. As a rule, the presence of an unlawful union security provision causes the decisions issued by arbitrators and by the board to differ to the point of being unreconcilable. Under a modified union security provision, the maximum number of possible situations subject to dispute arises. The areas of concern include an employee's waiver of exemption status, the financial makeup of union dues, enforcement of the maintenance of membership provision, and execution of the escape clause. 13 footnotes.