During the next four years disputes in Association plants were rare. In August 1890, a strike took place in Pittsburgh and, for the first time in the history of the industry, it was settled by a written trade agreement with the local union. This supported the idea of a national trade agreement between the two organizations. Since the dispute of 1887, negotiations with this object were from time to time conducted, the Defense Association invariably taking the initiative. Finally, the national convention of the union in 1890 appointed a committee to meet a like committee of the Defense Association. The conference took place March 25, 1891, and worked out a complete plan of organization for the stove molding industry. Every year two committees of three members each, chosen respectively by the union and the association, were to meet in conference and to draw up general laws for the year. In case of a dispute arising in a locality, if the parties immediately concerned were unable to arrive at common terms, the chief executives of both organizations, the president of the union and the president of the association, were to step in and try to effect an adjustment. If, however, they, too, failed, a conference committee composed of an equal number of members from each side was to be called in and its findings were to be final. Meanwhile the parties were enjoined from engaging in hostilities while the matter at dispute was being dealt with by the duly appointed authorities. Each organization obligated itself to exercise "police authority" over its constituents, enforcing obedience to the agreement. The endorsement of the plan by both organizations was practically unanimous, and has continued in operation without interruption for thirty years until the present day. Since the end of the nineties the trade agreement has become one of the most generally accepted principles and aspirations of the American labor movement. However, it is not to be understood that by accepting the principle of the trade agreement the labor movement has committed itself to unlimited arbitration of industrial disputes. The basic idea of the trade agreement is that of collective bargaining rather than arbitration. The two terms are not always distinguished, but the essential difference is that in the trade agreement proper no outside party intervenes to settle the dispute and make an award. The agreement is made by direct negotiation between the two organized groups and the sanction which each holds over the head of the other is the strike or lockout. If no agreement can be reached, the labor organization as well as the employers' association, insists on its right to refuse arbitration, whether it be "voluntary" or so-called "compulsory."